4 ISP connection, only 1 for specific outbound internet networks

I have a Big IP with 4 ISP connections, 3 of them are for internet access (works fine), 1 is for access to a specific networks on internet. How can I send tehe outbound traffic to specific internet networks only with one ISP, and the rest of traffic (internet access) to 3 ISP?. The configuratios is like:

 

Route default to internet with pool GW4: ISP1 IP, ISP2 IP, ISP3 IP, ISP4 IP. Pool GW: ISP1 IP, ISP2 IP, ISP3 IP Pool GW2: ISP4 IP VS for default access to internet: Source 0.0.0.0/0, Destination 0.0.0.0:0, fastL4, snat with one IP per ISP (ISP1, ISP2, ISP3), pool GW, iRule for specific networks with pool GW2. iRule for ISP4: when CLIENT_ACCEPTED { if { [IP::addr [IP::remote_addr] equals X.X.0.0/14] || [IP::addr [IP::remote_addr] equals Y.Y.Y.0/20] } { pool GW2 } else { pool GW } }