The A3000 and A4000 keyboards are the same just the connector is different. you can use A2000/A3000 keyboards on A4000, just use any cheap PC PS2 adapter.
I just refurbished the A4000 keyboard for myself and on the keyboards PCB shows A-2000 stencil on it, so it looks like all big box Amiga keyboards are the same, at least internally.
@blanghorst
I have couple misbehaving keyboards that need refurbishing, I may be willing to sell it to you after I fix some of them, but all of them are A2000 ones.
