I just posted this in response to another question, but cut 'n paste makes easy work of it.
The LS4 specs vary depending on the year. In '70 the LS4 was just an LS5 with a milder cam. It was only offered in the Big Cars that year. Torque was the same as an LS5 or 6 at 500 lbs./ft., but at a lower rpm (3000). Also it carried a large diameter single(!) exhaust. It is nothing to turn it into LS5 specs.
The LS4 disappeared for '71 but reappeared for '72 for use in Big Car wagons (Brookwood - Kingswood Estate). I'll have to dig out the cam specs some time, but for now I'll saw that it was nothing more than a single exhaust 454 rated at 230 net h.p. (vs. 270 for the LS5) & 360 net lbs./ft. @3200 rpm (vs. 390 @ 3200 for the LS5).
For '73 the LS5 wasn't mentioned anymore, it was just LS4. The Chevelle, Monte Carlo & Big Cars were rated at 245 net hp. @ 4000 rpm & 375 lbs./ft @ 2800. The Corvette was rated at 275 hp. @ 4400 & 395 lbs./ft @ 2800. The Corvette LS4 carried a forged crank with the 4 spd. only; all others got a cast crank starting in '73. Also starting in '73 the LS4 option was a dual exhaust engine.
