how much more can a solid axle with 4'' springs score on a ramp compared to a 4'' ifs lift?... is there that much of a advantige to the solide axle?

The number one reason a solid axle is better is strength and durability. There is no IFS set up that is as strong as comparable a solid axle.

It would depend on wheelbase, but assuming both trucks had the same wheelbase, the solid axle will generally articulate further than an IFS. Travel for an IFS in usually a few inches (except *** desert/prerunner race style suspension with super long A-arms and Porsche CV joints) due to the short length of the A-arms and stock style CV joints, where as a properly set up solid axle will flex twice as much (or more) than a IFS.

IFS is better suited for desert/high speed racing.