Trust me, I know what a 12th scale does with 4 cells, I drive 19T and mod.
But I also know what a pro10 does with 6 cells and a 4.5... dont you think a large 1/8 gass track is a bit different from the indoor carpet tracks 4 cell 1/12ths run on???
I've setup my old corally F1 to accomodate 4 cells this summer, but I'm pretty sure it'll have to be a fast motor, eg 5.5 or so to get some speed in the thing.
I'm also pretty sure that I'll get a run time of under 5 minutes with the 5.5 on 4 cells opposed to at least 10 minutes with a 6 cell.
4 cell is nice for indoor, really you wont hear me commenting on that, but not outside. You just need the extra rpms.
But hey, like I said, I'll give it a try with the F1 in a few months.
(LIPO on the other hand seems very interesting in my Pro10
)