Wow, congratulations on your unbelievably good buy, and welcome to the community!
Unfortunately that keyboard adapter you linked to will only let you use an A2000 or A3000 keyboard with the A4000, not a PC keyboard. They use the same style plugs (AT and PS/2) but they are very different in the way they work. If you would like to use a PC PS/2 keyboard you will need to buy a special adapter, either the Lyra (available at online Amiga retailers) or one of these adapters, which are the cheapest I know of:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=110723135382&ssPageName=STRK:MEWAX:IT
