maybe something like a XP Boost in the 4th slot? 10 coins for only 10xp a kill and 20xp a headshot, level 2 cost:30, would be somewhat 20 xp a kill 40xp a headshot maybe...
Damn, this is honestly not bad of an idea and I haven't heard anyone suggest this for the fourth slot. I feel like 10 coins might be too easy, maybe not...