I've never seen Socket A/Athlon MP boards for more than two processors
It is very unlikely that there will be another, faster Athlon MP model. The Opteron-line can do everything an Athlon MP can do (and much more) and provides a much more advanced and future-proof platform(Socket 940).
Most likely, the Athlon MP line will be discontinued in favor of the Opteron sooner or later.
In the 2nd half of 2005, AMD will release dual-core Opteron processors which will be compatible with today's Socket 940 motherboards. While it will still be awhile until those parts become available, using two dual-core Opterons on a dual Socket 940 motherboard would provide you with a lot more processing power than a (theoretical) quad-Athlon MP system could ever deliver....also, today's dual Opteron systems will easily outperform any dual Athlon MP solution.
------------------------- The opinions expressed above do not represent those of Advanced Micro Devices or any of their affiliates. <center><font color=red>MODERATOR</center></font> AMD Athlon 64 X2 6000+ 89W :: Asus M2N32SLI Deluxe :: 2x1024MB Muskin XP2-6400 DDR2-800 :: BeQuiet! Darkpower Pro 530W :: 2x WD Raptor 74GB/10k rpm, RAID 0 :: ATI Radeon X1950XTX