Your DHCP pool will only be matched for your primary IP address on the interface. In this case I suspect your X.X.X.X/24 is the primary address... and you don't have a DHCP pool for this prefix.
Try configuring 192.168.4.1/24 as the primary address on reth0.10:
set interfaces reth0.10 family inet 192.168.4.1/24 primary
Let us know if this solves your issue.
