Stonehenge is very doable in your time frame from LHR in a rental car.You would make it well before the 4.30pm slot and still have time for some dinner at a decent pub on the way back.
I often did the journey in less than an hour and a half but even a stranger could do it in not much more as the road between the two locations is direct with no towns to go through.
As millions of motorist who make the journey down to the West Country for the weekend could tell you it's actually at Stonehenge where the delays can occur at peak times such as Friday and Sunday nights.
The easiest route is M25,M3,A303.

https://www.google.ie/maps/dir/Termi...15!2d51.178882