7.4 is simply the liter designation of ANY 454ci engine.
As I understand it, all GenVI 454's are 4 bolt main.
I think the GenVI started in 96.
Not sure about the earlier variations.

I think all the base 454's (330's,300's,310's) starting in the late 80's came with 4 bolt mains. Reason being, I believe GM stopped producing the 2 bolt blocks. As well all the base motors will have cast cranks, cast pistons, 3/8 rod bolts, small oval(peanut port) heads. All Merc magnum motors got the forged guts, 4 bolt blocks, 7/16 rods, & rectangular port heads.

Gen 5 motors started around 1990, and Gen 6 in 1996. Even the base 454's can make decent power, you just have to watch your RPM.

LE

I had a 310 or 300 hp 454 in my boat and it was a gen 6 4bolt main.
It replaced a 300hp 350. Because the extra torque let me run a bigger prop and yet get on plane easier than the small block it picked up 3mph over the same hp small block.

What he said. My original 7.4L's ran at 4600. That was their peak rpm. The 7.4L Merc model group may go down as the most durable marine motor ever built. At 87 octane, you could not hurt these motors. Errrrrrr, well yes provided you don't run them out of water---------like I did.

Made them into 496 ci....

Believe I read that the 4 bolt mains began with the Gen V 7.4. They do have cast internals though...no forged goodies. Lot's of debate in the past about whether it was worth it to hop the motor up at all. I do recall reading an article on one of the other boards that was published several years ago that said with a decent intake, a set of Vortec heads and some cam work, you could safely squeeze over 400hp from the base Gen V motors without messing with the bottom end.