User blog comment:Xilinoc/Ch. 560 - Rages at Ringside/@comment-7079377-20131204201204/@comment-11051098-20131205201055

Good point. Ikkaku is probably on lieutenant-level, but as he keeps his Bankai secret, other characters in SS probably doesn't know his real power level.

@ RingotheDingo - Yumichika said that he doesn't like the shape of the number 4. He liks the number 3 the most, but since he couldn't be the 3rd seat as Ikkaku has that position, 5 was the number closest to 3 in shape, so he chose to become 5th seat :P