Yeah, to go with 4 cards on the board, you're looking at some higher end boards like the MSI 890FXA-GD70
because not that many have 4 full x16/x8 slots. Also, if you plan on putting the rig in a case, most cases come with only 7 slots on the back panel, so you'll need to buy a more expensive case as well.
With four cards though I wouldn't suggest going with a case and going with some sort open air setup and risers like this
If you do go with risers and want to go AMD I've had great luck with the Biostar A870U3
and it's pretty cheap right now at around $65. You can put that together with a Sempron and a GB or two of RAM for around $100.
If you have some older Intel LGA 775 chips, and can find either of these on eBay you can try either the ASRock P43DE3
or the Gigabyte P35-DS3R
All the boards I mentioned above I've been able to get four cards working on them with risers (or on the board itself in the case of the MSI) and windows.