Here is similar config with 2 internet links with equal speed:
http://wiki.mikrotik.com/wiki/Manual:PCC
In your case you will need 6 PCC rules that will divide traffic between 4 connections:
/ ip firewall mangle
add chain=prerouting in-interface=LAN connection-mark=no-mark dst-address-type=!local \
per-connection-classifier=both-addresses:6/0 action=mark-connection new-connection-mark=DSL1_2Mbps_conn
add chain=prerouting in-interface=LAN connection-mark=no-mark dst-address-type=!local \
per-connection-classifier=both-addresses:6/1 action=mark-connection new-connection-mark=DSL2_2Mbps_conn
add chain=prerouting in-interface=LAN connection-mark=no-mark dst-address-type=!local \
per-connection-classifier=both-addresses:6/2 action=mark-connection new-connection-mark=DSL3_4Mbps_conn
add chain=prerouting in-interface=LAN connection-mark=no-mark dst-address-type=!local \
per-connection-classifier=both-addresses:6/3 action=mark-connection new-connection-mark=DSL3_4Mbps_conn
add chain=prerouting in-interface=LAN connection-mark=no-mark dst-address-type=!local \
per-connection-classifier=both-addresses:6/4 action=mark-connection new-connection-mark=DSL4_4Mbps_conn
add chain=prerouting in-interface=LAN connection-mark=no-mark dst-address-type=!local \
per-connection-classifier=both-addresses:6/5 action=mark-connection new-connection-mark=DSL4_4Mbps_conn
More info on PCC:
http://wiki.mikrotik.com/wiki/How_PCC_w ... eginner%29
Also for failover create 4 default routes without routing marks. Set distance=1 and distance=2 for routes to 4 Mbps connections, and distance=3 and distance=4 for routes to 2 Mbps connections.