I'm gonna take a stab at this, because I had the same questions awhile back.
The ZZ4 is basically a traditional small block with a one piece rear main seal. It is also set up to run a factory hydraulic roller cam. It has the tapped bosses in the lifter galley for the lifter hold down mechanism, and the front of the block is machined for the camshaft retention plate, which keeps the roller cam from walking out the front of the block. As to parts interchangability, you must use a crankshaft designed for use in the one piece rear seal block, and the flywheel/flexplate that is designed for that crank. I think that is the major restriction. You can use a regular flat tappet camshaft, and you can bolt on any of the standard SBC heads, intakes, distributors, fuel pumps, oil pan, timing cover, etc.
The LT1 is considered to be the GenII small block. The major differences are that the cylinder heads are set up for reverse flow cooling, meaning that the coolant from the radiator goes from the pump to the heads first, which allows for higher compression without detonation. Standard SBC heads wont work on this block, nor will LT1 heads work on a standard SBC block. Another difference is the water pump, which is driven by the cam sprocket (I think!) instead of by a v belt. This is the part I'm not sure of...but I think the Lt1 did away with the distributor through the rear of the intake manifold and somehow has it running off the front of the crankshaft. I'm sure someone else will elaborate. I think the LT4 was a higher HP version of the LT1.
The LS1 is a beast unto itself - no real similarities to the old small block (except I think i read that the bore centers are the same).
If I messed any of this up, somebody PLEASE correct me.
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