Is Tama 8 or 9? Tengu mentioned her 8th birthday, but seemed to say that Ace came to Wano 4 years ago when Tama was 5, or was the 4 years ago reference separate from Ace's arrival?
There's a couple of things to bear in mind. Firstly, Hitetsu said "about" four years ago. Now, it's pretty strange that he'd need to be so vague when talking about a relatively recent event, but maybe by "about four years", he means "three years". I mean, to be fair to him (and Oda), it could've been right by the borderline of 3 or 4 years.
Secondly, let's say it was specifically four years ago - that's when Ace arrived, not when he left. So maybe he arrived 4 years ago when Tama was 4 but that scene where Ace mentions her age was a bit later when she'd turned 5. He would've have to stay long for that to be the case. It all depends when her birthday is within the year. So, to use real world dates for ease, let's say the current date in OP is 1st January 2018. That's Tama's 8th birthday. Ace arrived on 31st December 2014. That's 4 years ago, so it fits. He was there for a few days, during which it became the 1st January 2015, meaning Tama was now 5. So Ace arrived 4 years ago AND could speak to a 5-year-old Tama AND only had to stay a few days for that to be true.