According to the wiki page, the pips used in Star Trek: The Next Generation denote rank starting with 1/2 a pip (or hollow pip) and working up to Captain (4 pips) and even beyond into Admiral ranks. There are seven ship ranks assigned to pips: 1/2, 1, 1 1/2, 2, 2 1/2, 3, and 4.
Is there any in universe explanation as to why there is no ranking assigned to 3 1/2 pips? Wouldn't this be the perfect way to denote the First Officer? (For reference, Commander Riker wears 3 pips)