[name_m]Zachary[/name_m] [name_u]River[/name_u]!
i think [name_u]quinn[/name_u] [name_u]river[/name_u] or [name_m]beck[/name_m] [name_u]river[/name_u] goes best with [name_m]charlie[/name_m] and [name_f]lilah[/name_f] :3
[name_u]charlie[/name_u] is as good as unisex gets right now, imo, so [name_u]quinn[/name_u] would fit in nicely, while [name_m]beck[/name_m] has a bit more edge but is probably my favorite of the bunch!
I think [name_u]Charlie[/name_u] and [name_m]Jack[/name_m] make the most sense as brothers!
I’m swooning over your kids names! [name_f]Lilah[/name_f] [name_u]Winter[/name_u] is gorgeous. 
Of your choices I love [name_m]Jack[/name_m] [name_u]River[/name_u] and [name_u]Phoenix[/name_u] [name_m]Alexander[/name_m] the best.
I love [name_m]Jack[/name_m] [name_u]River[/name_u] and [name_u]Quinn[/name_u] [name_u]River[/name_u]!
I love [name_u]Beck[/name_u] [name_u]River[/name_u] and I think it fits with your other 2 children. [name_u]Phoenix[/name_u] [name_u]Orion[/name_u] would be my second choice.
I like [name_m]Jack[/name_m] [name_u]River[/name_u] and [name_u]Phoenix[/name_u] [name_m]Alexander[/name_m] the best! [name_m]Jack[/name_m] [name_u]River[/name_u] would match the naming style of his siblings better but I don’t think that’s a requirement. Many couples get more daring with their naming choices as they have more children!