Where is the fourth?

Notice E−pc at the end of the Special Relativity row above. E−pc (or, if you like, E⁄c−p) stands to momentum as SLOWNESS stands to SPEED. I noticed decades ago that E−pc would be more useful to consider for a system at high speed than for one at rest. In a rest frame, the quantity's magnitude simply equals the rest mass (rest energy). This is likewise as linear energy is more useful to consider for a system at low speeds than for one at lightspeed, where it equals magnitude of momentum (expressed relativistically). It's almost a mirror-image kind of thing. One needn't be a physicist, it's right there in the arithmetic. Anyway, now (2026-2-17 or so) I find it said by Google AI that E−pc is a formula for light-cone momentum and is useful to consider at high speeds. Call it "slowmentum"?—no, that already is defined for another physical quantity. How about "lentorment"? In doodles I used to label it lusk and symbolize it k. I'm not a physicist or a mathematician, but I was fiddling around with these things in search of Aristotle's Four Causes or the like. I think I found them and it's not something lame.

Assume relativistic units (i.e., lightspeed c = 1). Let "m" stand (as is now usual) for rest mass. p = momentum. E = total energy. e = linear energy. k = E−p (remember, c = 1).
E² = p² + m².
E = p+k = m+e.
p−m = e−k.
p−e = m−k.
√(2ke) = |p−e| = |m−k| = E−e−k.
Example:
◤ p=3.   ◥ e=1.
   ■ E=5.
◣ m=4.   ◢ k=2.
Note principles of Aristotle's four Causes at the corners.
◤ Agent as effector    ◥ Act as action (culminal end)

◣ Patient as matter   ◢ Act as form (structure, entelechy) (I'd say "borneness", balancement, as form.)
I used not to know that Ancient Latin patiens was a translation of Ancient Greek paschonta (undergoer, sufferer, e.g., "which is cauterized") with connotations like those of passion rather than of patience which are nearly the opposite, I assimilated agent:patient to make:let and must:can. So I had to make a fourfold out of the four causes' three causal principles (agent, patient, act).