This plot is the thing about “CB4,” a scattershot comedy written by Rock, Nelson George and Robert LoCash that takes aim at conniving groupies, mercenary Afrocentrists, right-wing politicians on anti-rap crusades and middle-class poseurs who think it’s hip to act like gangbangers. But sending up the macho, misogynistic postures of rap isn’t easy, because so much of the real thing already borders on knowing self-parody. Torn between celebration and sendup, “CB4” misses its big target as often as it hits. Still, it’s hard not to chuckle when Rock, in a slow-motion lovers-running-in-the-field montage, trips and falls under an excess of gold chains, or when he experiences a nightmare vision of his future in the “Hip Hop Retirement Home.” This isn’t a movie for anyone who thinks Ice-T is served with NutraSweet, but rap fans will appreciate its funky, insider’s irreverence.