To the best of my knowledge, the A4000 did not use a 15-pin VGA cable; only the A3000 had this due to the built in flicker fixer. To use a modern multisync monitor, your best bet is to purchase a flicker fixer board. Some models sit in the video slot, some clip onto the actual AGA graphics chip, and another actally connects a box to the external 23-pin port. These units 'double' the the video signal up to 30khz, the minimum for most of today's monitors. The external 23-pin port alone needs a monitor that will sync down to 15khz, which are pretty rare.
The Amiga uses custom keyboards and mice, although PC devices can be used with adapters.
As mentioned, Software Hut is a good starting point to check for this equipment.
:-D