Sadly the crystal ball is away for repair at the moment, but making a LOT of assumptions I am guessing you have tried something like this :
/ip firewall nat
add action=dst-nat chain=dstnat dst-port=8080 in-interface=pppoe-out1 protocol=tcp to-addresses=192.168.0.234
add action=dst-nat chain=dstnat dst-port=8080 in-interface=pppoe-out2 protocol=tcp to-addresses=192.168.0.234
add action=dst-nat chain=dstnat dst-port=8080 in-interface=pppoe-out3 protocol=tcp to-addresses=192.168.0.234
add action=dst-nat chain=dstnat dst-port=8080 in-interface=pppoe-out4 protocol=tcp to-addresses=192.168.0.234
where protocol is TCP port 8080 and you have 4 incoming interfaces, all being pppoe-out1-4 and your internal IP to forward to is 192.168.0.234.