Possibly the 500/4, but I think the 400/4 used a steel canister type filter (what you need is a paper element type)
If you're buying from a Honda dealer, quote a Honda part number 15410-426-010
A heap of other bikes use the same filter, and they all have their own part numbers. Here's a list:
MOTO GUZZI 65150000
Alternatively, buy a generic filter from a filter supplier. Here are some part numbers:
Hopefully you'll be able to find a filter amongst that lot.