Except that Thalmin's post:
+1 to each of two different stats at about every 4th level, but there are a few levels (I think 11 and 21) where every stat gets a +1 bonus.
Could also mean
4th (+1 to two different scores), 8th (+1 to two different scores), 11th (+1 to all scores), 12th (+1 to two different scores), 16th (+1 to two different scores), 20th (+1 to two different scores), 21st (+1 to all scores), 24th (+1 to two different scores), 28th (+1 to two different scores).
This results in one extra set of ability bonus increases to two of the player's choice.