[name_u]Max[/name_u], though its popularity has jumped in the past few years, is a classic. In the US it’s never left the top 1000. It can certainly sit well with siblings [name_u]George[/name_u], [name_f]Anna[/name_f], and [name_u]Daniel[/name_u].
If you call your kids [name_u]Georgie[/name_u], [name_f]Annie[/name_f], and [name_u]Danny[/name_u], then [name_u]Max[/name_u] could work. Otherwise, I’d go with [name_m]Maxfield[/name_m] or [name_u]Maxwell[/name_u].
[name_u]George[/name_u], [name_f]Anna[/name_f], [name_u]Daniel[/name_u], & [name_u]Max[/name_u] make a great set! As others have said, [name_u]Max[/name_u] is a classic name even though it’s been popular recently. Paired with a traditional middle name like your other children have, I think [name_u]Max[/name_u] is a great choice!
Another vote for [name_u]Max[/name_u]! We have a [name_u]Maxwell[/name_u] but call him [name_u]Max[/name_u] and [name_f]Anna[/name_f] is on our shortlist for a girl. We would use [name_u]Daniel[/name_u] if it wasn’t a close friend and [name_u]George[/name_u] is so classic (like Anna!) that he can fit in to many styles.