­Blood Lust + Thumbs Down
Hoist by my own petard, as one of the quintessential red buffs pairs up against a "Hey, look what the magic rules ought to support but don't" card.

So - when a creature is in combat; either its opponent dies, or it does. That works reasonably well with -4 toughness, and would produce bloodlust shifted into equipment. But we can go better by using deathtouch, I think.

Also, I really like the art on bloodlust. It would have been so easy to go for a crazed beserker; rather than a lady who is quite obviously literally in lust to a Wall of Blood. It amused me way-back-when too. So lets use another ambiguous name, and art of a blood-covered feather.

How about just "Equipped creature and all creatures blocking or blocked by equipped creature have deathtouch"?

­Barbed Battlegear did a slight toughness reduction on equipment; it's entirely possible that was conceived as an equipment version of Blood Lust. (So, of course, did Skullclamp, but that didn't mind if the creature died in the process; quite the reverse...)

I don't think equipment that can only go on a creature with toughness 5 or greater is a particularly sensible idea. Or at least, if this was printed it'd single-handedly make me recreate my old Magnetic Theft - Isochron Scepter deck of equipment designed to go on your creatures :P (Vulshok Gauntlets, Dead-Iron Sledge, Peregrine Mask, Skullclamp, and back when damage went on the stack, Sword of Fire and Ice. Sadly Grafted Wargear doesn't work.)

I wanted to recommend: "Equipped creature gets +4/+0, and its toughness becomes 1." But realized how silly that looks. Bah. I'd do it.