Are we assuming your shop floor is level? 4,000 lbs is still 4,000 lbs even if its on dollies. Getting squished between the truck and the bench would pretty much put a damper on the day ...
Size depends on how much your truck weighs. It seems to me to size the dollies, you would take the weight of the truck and divide by four. Throw in a little extra capacity to account for the front of the truck weighing more than the back.
I would think those 1,500-lb dollies of Tim's would do the trick for most pickups.
I had a set. I used them for a while and then got rid of them -- they were in the way more than they were useful. They don't store handily.
John
