This is how i would do it. should work. Maybe there is a better way?
First make sure Bridge -> settings -> Use ip firewall is enabled.. disable any ports on the bridge
Setup a DHCP server on ETH3 and ETH4 different ip/subnets (for example 10.0.10.0 and 10.0.20.0)
set ETH3 ip address to 10.0.10.1/24
Set ETH4 ip address to 10.0.20.1/24
Setup DCHP Servers & pools for each subnet with the addresses you want it to hand out and map them to the appropriate ETH ports
Setup NAT in the Ip -> Firewall NAT (Chain=srcnat src-address(10.0.10.0/24 and 10.0.20.0/24 action=Masquerade) for both ETH3's and ETH4's subnets
Setup MANGLE Rules for Router Marks for each subnet Ip -> Firewall -> Mangle -> chain=prerouting src-address=10.0.10.0/24 & 10.0.20.0/24 (two seperate mangle rules) Action=Mark Routing New Routing Mark=VOIP / DATA
Setup Routing. Make two default routes (0.0.0.0/0) in IP routes in Ip -> Routes and select routing mark VOIP then create the second route and set the routing mark DATA. Set the gateway of each default route and packet mark route according to where you which dsl modem you want it to go to (ETH3 or ETH4's IP addresses)
setup your ip's for each DSL ether connection (Either static, or DHCP client on ETH1 and ETH2)
Make any sense? its late.