Personally, I think 4.5 days sounds like a nice relaxing trip to me. I could easily do 1 day at each park, 1 day in the HP sections, 1 day at a water park, and .5 a day at Citiwalk or at the resort. However, we tend to experience about 75% to 90% of the rides and attractions at least once when we go, our favorites more than once, and we eat at least 1 nice sit down meal a day. We hardly ever skip anything unless the weather is too cold for the water rides. We also spend a good deal of time on non-attraction entertainment such as: window shopping, nice meals, snacks, street performers, character meets, taking pics, spraying people on the raft ride (a must do for every trip), exploring the HP area (sending owl mail, playing with interactive wands, etc.), and any special concerts or parades going on. It is the one thing I love most about Universal; we don't have to rush around unless I plan a 1 day trip. We can just take our time, relax, and still fit everything I want to do in. I don't have to make the choice between staying and watching the acrobats we stumbled upon or experiencing the attraction we were on our way to because I know we can fit both in.
Now, will 4.5 days be too much for you? Hard to tell without knowing what all you plan on doing. It is impossible to say how many days are too few or too many as each guest tours differently. Are there enough things to do at Universal (Citiwalk, resorts, parks) to keep you busy 4.5 days? Yes. Will you want to do them all? Probably not. Will you find enough? Only you can answer that.
My advice would be to make a list of what you are interested in doing at each park and at Citiwalk. From there, you can estimate about how much time those attractions are going to take you (travel time if applicable + attraction run time + wait time). Wait time will be fairly low end of Sept about 10-15 mins off hours and less popular rides to 30-1hr headliners and busiest parts of day. Less if you're staying on property and can skip the lines, estimate about 10-15 for everything. You can also simplify the equation by simply saying each park takes about 1 to 1.5 days to see everything once if I tour open to close. I want to do half the attractions at 1 park, so I need .5 to .75 of a day open to close. This would translate to 1 day for someone who sleeps in, arrives late, and then leaves for dinner in Citiwalk, so there's my .5 to .75 of a day. Also, don't forget to factor in meals if you plan on having any sit down ones (1-2hrs per meal for the waiting and eating). Estimating like this isn't exact, but it does give you a good general starting point to work from. You can usually figure out if you need 1-2 days verses 2-3 and so on this way.
If you are looking for some ideas in case you run out of things you want to do in the parks, I'd look into exploring Citiwalk and the resorts. You've got mini golf, bowling, a movie theater, the Blueman Group show, themed restaurants, shopping, and pools and hot tubs to relax in.
As far as a car goes, I wouldn't worry about renting one unless you plan on using it a lot. It isn't easy to get around in general without one, but if you decide you want to go to another them park like Disney or Seaworld, there are a few ride share companies out there that specialize in taking people to and from the different theme parks, especially to and from Disney for a reasonable price (definitely less than renting a car). So, if you get there, and do decide that 4 days is just too much, you can always arrange for a ride share / shuttle to Disney or somewhere else for the day.
