Many four year old's speak incessantly at this age- even when they DO get all the attention. There isn't a lot to be done about it. Having said that, mine was the same way and here is what we did to help us adults maintain our sanity amongst it all:
Make sure he gets social time with friends - for us that was time in theater classes and plays. Try to get him involved in things he does because he loves them. For some it might be time at a pre-school or involved in a sport. The activity time is much more of a time-off from listening for you than a stopping him from talking incessantly measure - he will have stories to tell you about her social time when you do hook back up. If you can get a family member or even trusted teen to take him to his activities that will make this "time off" even more valuable for you.
Make time to listen to his stories - We had family meals where we specifically asked about Alice's day, what she had done and learned, what was the low-light, what was the highlight etc. She knew this time would come and that we would listen, paraphrase, ask clarifying questions and summarize (ways to show you really are listening) when it did come. I also did this in the car with her between activities for a few minutes after each activity, at the end of the day before bed for about ten minutes, and at the beginning of the day I always asked about her dreams. There were also plenty of "unplanned times" that I listened too. A sister with Cerebral Palsy adds a big complication to this, but it is still important that the time be given. Perhaps you and your partner can take turns.
Stress turn-taking - we listened well to her during those tale conversations, but she was expected to do the same for her parents as well. Everyone got a turn AND modeled good listening. This will probably be especially helpful in your house. When the sister's needs are overtaking his, perhaps he can write a reminder note (it doesn't have to be legible) to give you that there was something he needed that he is waiting on. If he trusts you will get to his need too, he can certainly learn that someone else is having a turn right now.
Teach "needs of others" - Because I knew I made plenty of time to listen to her throughout the day, was connected and made time for fun activities, reading books and other things, I felt no guilt or remorse about giving her a request for quiet at times too. "I am a person that needs quiet time everyday to recharge my energy - it is like charging batteries for me. Can you give me a little of that time right now? I need about five minutes." Obviously, for you guys, needs of others is a big lesson being learned anyway, the thing to be careful about is that his needs are important too, but that everyone needs things and not everyone's needs are equal.
Incorporate Quiet Time into your Day - he may not need a nap any longer, but he can still have "quiet time". Alice had about 30-45 minutes of this everyday. This was time for a nap if one wanted one, reading, doing mazes (she liked mazes a lot so this was her activity).
For your specific circumstances it might also help if your son can take an active roll in attending to his sister's needs as well. You may find he is a great help and that his sense of self worth and confidence grows with his sense of responsibility. If he can tell you stories and talk about thing WHILE you are both helping his sister, you've been able to do something wonderful for both your kids at the same time.
I also hope you have some help so you can still get some time off for yourself and aren't forgetting to take care of yourself too. We are never at our best as parents if we can't care for our own needs too. Especially in your stressful situation, I'd imagine you need this time and space more than most. Is there a family member nearby or a local church group that can help in providing both of you some time to yourselves - even together to yourselves once in awhile?
I don't know where you are located or how it all works, not having faced the challenge you face, but this link will take you to the Cerebral Palsy Support Network which may be able to provide some support, ideas or other assistance. If not, maybe there is a similar network that CAN apply for you.
Isn't it funny how we strive for that first word, listen for it, cheer it on and are SO excited when they start talking and then immediately start reminiscing about the days when the talking stopped once in awhile?