[Greg 1]

A lot depends on the sort of cinematic abilities related to gunpowder that are available and what TL of guns the PCs can have.

TL 4 matchlocks, governed by realistic rules, aren't going to unbalance anything. On the other hand a PC with the Sharpshooter advantage and an Uzi probably will.

Three approaches strike me.

One would be making TL 4 firearms freely available, but not allowing cinemataic skills or abilities involving them.

One would be designing a dungeon fantasy character class around the cinematic use of TL 4 firearms. The trick is giving them an interesting niche alongside the bow-wielding Scout. Perhaps they trade RoF, reliability and some cinematic abilities for heavier damage.

One would would be designing a dungeon fantasy character class around the use of higher TL firearms (if other planes exist, perhaps a gunslinger or marine has slipped in from another universe).