[name_u]Rhys[/name_u] for me! [name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Rhys[/name_u] makes a stunning sibset, and I adore Rhys/Reese in general.
[name_m]Knoll[/name_m] would be my second choice… it is cool since it’s a family name, but I’m having a bit of a time imagining [name_m]Knoll[/name_m] on a real, live person… it just makes me think of the synonym for hill. A few others I think would work really well with your other kiddos:
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Cecil[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Reeve/Reid
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Lorcan[/name_m] (I am a HUGE fan of [name_m]Lorcan[/name_m], hahaha)
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Desmond[/name_m] (like this one so [name_u]Rowan[/name_u] wouldn’t be the only one with a two-syllable name?)
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Oscar[/name_m] (not very Gaelic, but I like it with your other three!)
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Morgan[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Milo/Miles
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Fergus[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Magnus[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Adair[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Arthur[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Balfour[/name_m] (love [name_m]Balfour[/name_m] with the nn [name_u]Rory[/name_u], but idk if [name_u]Rowan[/name_u] and [name_u]Rory[/name_u] is too similar in your sibset? You could also do [name_u]Alfie[/name_u], I suppose…)
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Brady/Brodie
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Calder/Callum
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Caedmon[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Conan[/name_m] (!!)
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Cormac/Corbin/Mackenzie
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Cullen[/name_m] (!!)
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Daley[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Devlin/Devon
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Ewan[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Fife[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Galloway/Calloway (Calloway would be fun with the nn Cal?)
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Grady[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Guthrie[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Ciaran[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Cillian/Killian
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Conor[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Lachlan[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Lennox[/name_u] (so cool!)
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and Munro/Monroe
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Quillan[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Roarke[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Sullivan[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Torin[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Idris[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Ellis[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Macsen[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Caio[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Price[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Rhydian[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_m]Ianto[/name_m]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Taliesin[/name_u]
[name_u]Rowan[/name_u], [name_f]Maeve[/name_f], [name_u]Finn[/name_u], and [name_u]Taran[/name_u]
Would also love to recommend [name_u]Bryn[/name_u] but I feel like [name_u]Finn[/name_u] & [name_u]Bryn[/name_u] as brothers doesn’t work great. 
Good luck!