I think 4.5 is a little young for slot cars.... we are getting them for my son this year and he's 7.5 (and his brother is 6) and I think that is more age appropriate.
We've been in car mode for well... 7 years :)
We have the Plan City wooden roads - they hook with our wooden (Melissa and Doug) train track and my Mom also bought the boys the parking garage for Christmas a few years back. They love it. The roads were a big hit - and still get used daily (now they use them with lego's too).
We have a number of Hot Wheels tracks, but honestly, my boys don't use them much - maybe other kids would be into them, but I wouldn't recommend them, especially if you already have the wall tracks.
We got a play mat for x-mas two years ago that was a vinyl map of our neighborhood that was pretty popular for a while... my SIL bought it and I suspect this is where she got it - very reasonably priced...
http://www.etsy.com/listing/63549128/your-neighborhood-cu...
Here is the Plan City Road set we have...
http://www.amazon.com/PlanToys-PlanCity-Road-System-Delux...
and parking garage...
http://www.amazon.com/Plan-Toys-Series-Parking-Garage/dp/...
Good luck!