The M.U.S.C.L.E. four packs contained four figures (of course), either pink or colored, depending on the time it was manufactured. While the 28 packs contained figures not available in four or ten packs, the four and ten packs had figures that weren't in the 28 packs (such as the ever-elusive Hand). Since the M.U.S.C.L.E.s in the ten packs were in a semi-transparent garbage can, the four packs were the only way to know for sure what you were getting. Most MUSCLEheads remember that back in the day these MUSCLE four packs sold for roughly a buck.
There were three different color assortments for the four-packs: all four pink (the initial assortment), two pink and two colored, and all four colored. The card back had a cartoon featuring Muscleman and Terri-bull as well as an offer for the M.U.S.C.L.E. poster. See below for detailed scans.
All-Pink Four PacksThe initial assortment contained all pink MUSCLE figures. You'll notice on the back of this four pack is the MUSCLE poster offer. However, some of the all pink four-packs did not have the offer. Presumably, those packs were shipped before the poster was available. The card back for the all-pink packs follows.
2-Pink, 2-Colored Four PacksThe next wave featured two pink and two colored MUSCLEs (the colored figures are always either red, dark blue, or dark purple). Why the half and half four-packs? My assumption is that Mattel produced the mixed red, blue, purple and flesh 4-packs to slowly introduce colored figures on the market. Instead of producing all available MUSCLEs in a bunch of different colors, which kids may or may not like, Mattel may have decided to test the waters by providing a limited production of colored MUSCLEs in just a few colors: red, blue and purple. When Mattel determined that the colored figures were a good idea, they then expanded production by increasing the number of colors available.
Another note: the poster offer was still on the back of these cards, but presumably the poster offered was the colored one.