This might be a good place to start:
http://www.opticsplanet.net/how-to-choose-riflescope.html
Some other interesting reading might include:
http://www.chuckhawks.com/under-scoped.htm
http://www.chuckhawks.com/thoughts_scopes.htm
http://www.chuckhawks.com/straight_scope.htm
Your $400 will buy some incredible glass these days, when compared with that of even 5 years ago. And "4x" and "Whelen" sound (to me, anyway!) like a match made in heaven!
If you take even a cursory look around, you can find some awfully good deals on the 'net. However, I'd still urge you to doyour comparing with your
own eyes. Lots of folks have some well-reasoned opinions, but
you are the one who'll have to look through the scope every time at the range or in the field. So you may hear people universally RAVE about a certain scope, but if you can't find a store that has one you can compare side-by-side with other contenders, I'd think more than twice about it.
Hope this helps,
FC