Unfortunately it's not quite that simple. Your motherboard is a socket A mobo, meaning that it supports AthlonXPs and Semprons (Semp's are new budget CPUs from AMD).
The new Athlon64 range, which is their new "high end" series, which inlcudes the Athlon64, Athlon64 FX and Athlon64 X2 are all either Socket 754 CPUs or Socket 939 CPUs, meaning they are not compatible with your motherboard.
The fastest CPU you can get for your current board is an AthlonXP 3200+, because that's the fastest Socket A chip they make. It doesn't run at 4Ghz, but neither does any consumer CPU on the market yet, unless it's very overclocked.
I'm in the same situation because I have an XP2800+ and want to upgrade, but it means I have to upgrade my mobo and, probably, my graphics card at the same time.