This list assumes you are using child friendly and organic cleansers.
Depending on the type of vacuum you have, he might be able to help with that. If you have an upright it will be too much but cannister vacs mean the part the child is moving around a lot are lighter weight so he can do that.
You can also spray a mirror and have him wipe the bottom while you do the top - same with windows and glass doors.
matching socks, sorting clothes into piles of mommy's clothes, daddy's clothes, his clothes and so on. Folding easy fold items.
If you do gardening, watering plants was already mentioned, but Alice was helping with weeding by the time she was four. I started by having her point out plants she thought were weeds and then I would pull them (scavenger hunts are so much fun). THEN when she got a pair of gardening gloves and a weeding tool one spring in her Easter basket she was super excited to use them and now sometimes if she is out in the backyard playing I'll peak out and see her weeding even without me.
Taking care of feeding pets (reminders will be needed).
helping to set the table.
measuring and stirring ingredients in the kitchen.
picking up general "stuff" around the house.
taking out the compost and stirring it in (with help).
wiping sinks and faucets.
Cleaning the bath-tub with you.
Washing dishes that are relatively light weight and not sharp with you. (You wash, child dries).
Emptying a dishwasher of all the dishes that belong in cabinets the child can reach. Instruct your child to let you handle knives and particularly fragile items.