To answer this, I am going to meander a little, this may seem irrelevant at first, but I hope you'll see it threads together. I have not focused so much on step by step solutions, but as a holistic approach to your situation and the complex nature of it.
Firstly I would like to give you some details of our family history, as in your situation, I have found it helpful to have identification with people in similar situations. That feeling of being singled out, isolated and overwhelmed can be a little relieved when we know we are not alone in being so alone.
I can really identify with your situation. We have a sort of reverse situation. My children lost their Dad a long time ago (ten years).. and about 11 years ago I was diagnosed with a disabling and life threatening illness (rapidly progressing systemic sclerosis and severe recalcitrant recurring polymyositis). So in some ways I have had your challenges and to a much lesser extent your wife's challenges.
When my two youngest were babies, my muscles were so wasted I could not lift them, in fact I could not roll over in bed. I was undergoing chemotherapy during this time when my husband committed suicide. Little were we to know that this was to be the beginning of seven years of chemotherapy and other immunosuppressive treatment, in excess of 130 hospitalisations, many ambulance trips, surgical procedures and close calls. I can safely say, the past decade has been a living nightmare or hell like experience, that only people who have experienced such surreal trauma like conditions can relate to.
Dealing with such a multifaceted dynamic, you are dealing with your, your children's and your wife's grief. This is an ongoing thing, not something that has an end point. It is chronic and difficult to resolve.
I can only guess that you would be taking on much of the day to day care of your wife, as you have expressed you are in love and would probably want to do this. This can lead to exhaustion, adding to already depleted energy supplies and time constraints, as the sole provider and carer for the family.
Society is not well equipped to support people with chronic ongoing grief. It is a little understood area and people, frequently, expect time to heal everything, when indeed, over time, situations and distress can be exacerbated.
Changing your thinking.
Trying to fulfil the role of both Mum and Dad can really do your head in (I know from experience). You don't have that softer feminine person to bounce your traditional authoritative Dad stance off. Unfortunately there is no easy way around this, and requires a lot of extra communication with your son to explain how you bounce between the roles. It's like being good cop and bad cop in one.
Being four, your son is old enough to start grasping simple concepts, and it doesn't hurt to start discussing these things now, as it will help him cope as he grows up and will form the basis of firm communication between you both. When I am being (what my children perceive as) harsh and strict, I have to explain to my kids that, I am the Mum and the Dad, and it's my job to protect them, as well as nurture them.
I am finding that I have to maintain the harder line, disciplinarian role more than the nurturing mother role. As they will push the boundaries like crazy, and being a solo pilot, I am having to fly a tighter ship so to speak. As a sole parent, and a Dad, you may also find yourself, as indicated, taking a firmer role and your son having a firmer upbringing than with both active Mum and Dad, but this is not necessarily a bad thing. This is where the communication comes in. You can stop and take stock, pat his head or scoop him up in your arms and tell him how much you love him.
At the end of the day children thrive on strong, clear, healthy boundaries and love. Love without the boundaries is as toxic as boundaries without love (my opinion). Having a predominantly male parenting role is not a bad thing, just an experience.
My point is, you may well find there is less you need to change as a parent, and just some adjustments in your thinking.
Children are resilient, having said that, they do grow up and carry unresolved burdens. You are setting your son a brilliant example of how to be a loving, responsible parent and a partner. You love your wife and it shows, you love your son and it shows. These are things that are more valuable than worrying about fulfilling a certain role.
There are three practical ways to ameliorate having a male parent as a sole parent.
1. Affection. Stop and remember to cuddle. Never stop telling your son that you love him. Even as a man. These are things more readily available from a Mum in a day to day setting.
2. Emotion. Do not hide your feelings from your son. As parents we, naturally, tend to hide our emotional difficulties from our children. I have found it is a good thing to sometimes let our children see us cry. Especially as a man, when it gets too much and you feel despair over what has happened to the love of your life, let your son see you cry and tell him why you are sad. This is something that children are more likely to see with their Mums, it's important that you allow yourself to show some of this to your son (not every day, so it is a burden for him, he needs to feel you are strong and can cope, but that it is also ok, to break down at times and feel weak and vulnerable).
3. Female role models. I have found male role models for my children, without becoming involved in relationships. This is important for you also, as your wife is still there. If there is a grandmother or aunt, neighbor or close friend of your wife's that can be this. Otherwise it is a matter of finding (not creating, it will fall into place) an appropriate person through some activity or interest that your son has.
In my children's case we had a shortage of male role models (my brother died the year after their father) and it has become Martial Arts. My oldest son now teaches it, he has his mentor (a black belt) and as a mature 19 year old, he has become a great role model for his 13 yo brother and 12 yo sister. There has not always been an appropriate role model, but I believe it is better to have no role model than a bad one.
For what it's worth, I think that your son will be fine. With a father who cares enough to post here and the person you are, things have a way of working out. I can say this to you as someone who has travelled many dark years, and we still have our moments.
It's important to be mindful that this situation will develop over time and you may not find yourself coping so well at times and it is important to forgive yourself for being human.
One thing you need to be mindful of is caregiver burnout. The following quote from WebMD is good and will also help you to identify much of what you are feeling, as normal for your circumstance. I think the point about Role Confusion also spills over to your parenting role.
What Causes Caregiver Burnout?
Caregivers often are so busy caring for others that they tend to
neglect their own emotional, physical, and spiritual health. The
demands on a caregiver's body, mind, and emotions can easily seem
overwhelming, leading to fatigue and hopelessness -- and, ultimately,
burnout. Other factors that can lead to caregiver burnout include:
Role confusion: Many people are confused when thrust into the role of
caregiver. It can be difficult for a person to separate her role as
caregiver from her role as spouse, lover, child, friend, etc.
Unrealistic expectations: Many caregivers expect their involvement to
have a positive effect on the health and happiness of their loved one.
This may not always be realistic.
Lack of control: Many caregivers
become frustrated by a lack of money, resources, and skills to
effectively plan, manage, and organize their loved one's care.
Unreasonable demands: Some caregivers place unreasonable burdens upon
themselves, in part because they see providing care as their exclusive
Other factors: Many caregivers cannot recognize when
they are suffering burnout and eventually get to the point where they
cannot function effectively. They may even become sick themselves.