NCR had a couple of SMP 486 boxen (NCR 3450, 3550 & 3600) that were supported by SCO Unix, SVR4 and (early) NT. I think they had support for OS/2. ALR, Unisys and Compaq made somewhat similar 486s with roughly similar OS support (although I vaguely recall the Unisys stuff was OEMed elseware). The support problem was 486 predated standardized SMP hardware (APIC), so it was all custom hardware.