That's a Metri-Pack 480 and here's enough information to get you through un-pining it. You don't need a specific release tool, just something that's very similar. Your connector should be 3 populated and 1 plugged I'm quite sure.
http://catalog.delphi.com/connectors...ure/met480.pdf
Cross-Sectional view is a "tell-all".
If you've bought a 4 pin connector and would like a correct cavity plug let me know.