It won't. Put that $30 towards opening the trans back up and fixing the real problem.
Usually the lower gear synchros are the ones that start grinding first. A fourth gear grind is not normal at all, esp. if 2-3 don't do it, esp. if it was just rebuilt. There is no fix in a bottle here, where a minor 1-2 or 2-3 grind can actually sometimes be made SLIGHTLY better or worse, esp. in the cold temps, w/ different fluids.
Pennz synchromesh has worked better for me than RP in the applications I've tried it in (nv3550, nsg370).RPs marketing/distribution is superior to many of its products. A lot of their products have poorer specs than most (if not all) competitors products.
'94 YJ - SBC/AX15/231, SOA, 8.8, 34" swampers
'06 LJ Rubicon - 31" ATs, OME 2.5", belly-up skid, other junk
'75 CJ5 - 232/T14/D20, 'glass tub w/ 2" BL, otherwise stock