RE: IM6 vs. IM7
There is little to no different between 2 and 4 pc rods for most manufacturers in terms of feel or ability of the rod. It takes a very trained caster to tell the difference in most cases and most of the time it is a difference in taper felt, but that the 4 pc rod has more ferrules or a worse feel, just different. You should have no problems with a 4 pc rod.
IM6 and IM7 were originally the trade monikers of the prepreg graphite sheet as sold to the blank manufacturers. There is debate about what IM stands for and no one has a definitive answer. At first, the IM numbers were used to indicate the 'quality' of a rod. The higher the number the better the rod was suppossed to be. It was marketing hype though, since rod performance is a combination of prepreg sheets, scrim, taper, and processing. Now IM are used in the same way, but with a lot less emphasis by the major manufacturers. The private label and store brand rods still use it to an extent to sell rods.
Typically when you see IM6 you are talking about graphite scrim with a young's modulus of 36-41 million PSI. IM7 typically is 41-48 million depending who you ask. Again, IM doesn't really mean much. Given the same scrim, taper, and processing an IM7 rod will be lighter for the same stiffness or stiffer for the same weight. But considering how close they are in properties and the differences in all other considerations the graphite itself will not be the cause of any differences you see.