Will this work?

Maybe. I had an Asrock board that refused to post when only using the one P4 connector. Had to get another PSU, then the mobo worked fine ever since.

I think it would also depend on the CPU you're using. The AM3 quad and hexacores would likely need the extra juice, esp black edition CPU's.

The was a post on the forums the other day about thuis issue. I cn't find it now but here's a link to another discussion:
http://www.tomshardware.com/forum/289500-28-power-connector-okay
 
I replied to your {duplicate post} http://www.tomshardware.com/forum/290172-30-connector

If the 4 of 8 pin CPU power is capped then on that CPU you'd be fine. You can purchase a 4 to 8 pin adapter, or obviously a new PSU if there's any problems. Some MOBOs with uncapped 8 pin 'may' have issues using the MOBO's phase power correctly and in that case get the 8 pin adapter:
12-201-005-02.jpg

http://www.newegg.com/Product/Product.aspx?Item=N82E16812201005