Block vs Heads vs Inlet - interchangeability?
 

How much can you mix n match block/heads/inlet? Can you throw together a pair of STi4 heads, a phase 1.5 inlet manifold, with either an early closed deck block or a brand new EJ257 and have it just plug in and work on a v4 ECU? I'm assuming the exhaust ports underneath are all the same?

You can mix and match any block with any head although some may need some fettling/porting to make the most of them. It's when you try and mix up heads and inlets you'll have problems.

The above mix of parts you mention would work just fine..

So is it true that v5 heads won't fit on a v4 block?

They will physically fit and bolt on the block. But the manifold bolt holes are different than the v4 heads, but you probably know that.