FASA Games, Inc.

If spell lasts 4 rounds and in Spellcasting test character achieves 3 successes (+4 to damage test) is that bonus added in all rounds the spell lasts?

Yes.

Thanks.

Out of curiosity (since right now I can't be arsed to check)... how many of those kinds of spells are there?

From a design standpoint, most damaging spells with an extended duration shouldn't give bonus damage on successes.

Only one for Wizard is:
Flame Flash
Duration: 2 rounds
Success Levels: Increase Effect (+2 Effect Step)
Extra Threads: Increase Duration (+2 rounds), Increase Effect (+2 Effect Step),
Increase Range (+10 yards), Additional Target (+1)

Suffocation, Stone Rain are - Success Levels: Increase Duration (+2 rounds) not (+2 Effect Step)...

True, but they do combine the Increased Duration + Increased Effect mechanics, just not quite in the same way. So the question still applies.

Yeah, but the point with those (and most damaging spells with an extended duration) is that the increased damage would be intentional as a result of extra threads, and not the result of exploding dice.

While the 'net effect' kind of works out the same, there is a built-in limit to the former (from the way extra threads work) that isn't the case with the latter (an especially good die roll can cause unexpected results).