A quick look at Newegg doesn't show a single LGA775 board with 4 RAM slots.
They have this odd board, though:
Newegg.com - MSI G41M-P33 Combo LGA 775 Intel G41 Micro ATX Intel Motherboard
It isn't really a 4 slot board; you can use 2 DDR2 DIMMs or two DDR3, but not both at the same time.
Socket 775 boards seem to be getting scarce, if purchased new.
I'd probably consider an AMD FX-4100 and an AM3+ board if trying to build a new budget system, but just those together would be not much less than $200US. I presume that you are looking for more like a $50 upgrade.