Each R is worth 10xp per Skill Level.

Each SR is worth 35xp per SL

In order to get a SL 4 SR weapon, worth 140xp, you need a SR and 6 Rs (95 xp total).

So you trade 95xp for 140xp. Raising the SR to SL5 would be a waste though. You'd spend 40xp (4 Rs) for only 35 more xp.