Frankly, I'd try using standard 2 pin din from the 2400 into the 4 pin din socket on the 4500's.
The 2400 won't accept the 4 pin Speakerlink plugs... The 4500 will accept 2 pin plugs.
You won't have any display... but I am not certain the 2400 would even push display data in the first place???
I am also not certain about any triggering that may be required... I suppose you could disconnect the amp in the 4500's.
If it were me: I'd look for a BM4500... That way you would have the full boat... a well matched system w/ full display via Speakerlink. Then get an old set of single fret passives (S45) for the 2400, or sell it off to fund the BM4500.