If I combine a +3 bow like Tansheron with +1 arrows (or for instance Tuigan +1 bow with +3 arrows), is this equivalent to hitting with a +4 weapon?
If I combine a +3 bow like Tansheron with +1 arrows (or for instance Tuigan +1 bow with +3 arrows), is this equivalent to hitting with a +4 weapon?
It will give you +4 to hit on the die roll, but you still won't be able to damage creatures requiring a +4 weapon to hit them (like Kangaxx for instance).
For launcher based weapons (slings, bows, crossbows), it's the AMMO that determines what you can hit - the actual launcher is irrelevant (except for launchers that don't require ammo, like Firetooth Crossbow). So to hit Kangaxx for example, you need +4 arrows. No +4 arrows in BG 2 (not even ToB either). For example, Ravager in ToB requires +4 weapons and ammo to hit it - thus a plain sling with +4 bullets would let you hit it.
Therefore, for Tuigan's bow, you need to load the bow with the appropriate ammo for what you trying to hit. Fighting a stone golem? You need +2 arrows, etc.