Michigan said:
You can not take the 4 year old with you on the
ecv if that's what you are thinking about.

First of all, the park rental
ECVS and most of the off-site rental places specificially have in the contract that only one person (over the age of 18 years) can ride on the ECV.
Second, it's not safe. A small child on the lap of the driver can easily be in the way of the driver operating the ECV (even if they sit perfectly still). They can also inadvertenly hit into the driver's hand or controls and cause the ECV to go where the driver is not intending it to go. I've seen that happen, and luckily no one got hurt, but could have.
If you had your own ECV, it's your liability, but the rental companies are not willing to take that liability. Even if it is your own ECV, your homeowner's insurance may not be too happy to pay out on a claim if the accident was related to a child riding with you.
If you are not asking about the child riding on the ECV and just wonder how you will keep up with the 4yr, first of all, I'd suggest renting from one of the off-site places. The ECVs are smaller, have a greater range of speed and many people feel they are easier to operate than the ones you can rent in the parks. Also, if you rent from off-site, you know you have one and you will have it when you are at the resort as well as at the park. It's possible for the park rental ones to be gone when you get there and then you would have to rent a wheelchair and try to push it.
I think whether you can "do" WDW by yourself with a 4 yr old depends on your 4 yr old. If you can count on him/her to stay with you, not get in the way of the ECV, it could be doable. But, many people find driving an ECV for the first time to be a daunting task in itself. Driving one for the first time with a 4 yr old would be harder.
If you are going soon, the crowds should be much lower than they were in the summer, which will help. I don't know how much you are able to walk around, but if you can, you might want to use the ECV for distances and park it when you get to an area and then walk around. You might want to see if you can get another adult to go with if possible.
Other things to think about besides an ECV are using the child's stroller as a support. Many people with knee or hip problems find that pushing a stroller gives them support so they can walk more easily. My FIL sometimes used to push my DD's wheelchair for that reason. And, taking things slow, lots of rest periods and not trying to do too much will help too.