Serial Interface Link speed is 2 Mbps
Ethernet interface link sepeed is 4 Mbps
The idea is to bundle the two links using Multilink PPP thus make a virtual thick pipe.
how could multilink PPP realize that second link is faster?
How to deal with different MTU if PPPoE were used on ethernet?
multilink PPP works on ISDN B channels (all at 64 or 56 kbps depending on how it used)
or it works on multiple serial interfaces of same speed.
Use EIGRP if it is a Cisco only environment with variance or do the following if possible:
divide and conquer approach:
if Vlan subinterfaces can be used on ethernet define two vlan subinterfaces that you use as two routed links assign a bandwidth of 2000 kbps and you can use OSPF or RIP or ISIS on the three links with no MTU problems
At the end some solutions may appear interesting but this is too troublesome it does not pay off.
consider that modern CEF supports per packet load balancing that would be similar to ML PPP load balancing.
per packet load balancing is not recommended and you should be happy with flow based CEF.
Hope to help