"I have no idea why there are two groups and I want to remove one if possible, hence my question."
Well one possible reason for two mHSRP groups on the same subnet could be for gateway load balancing, especially before GLBP. With the advent of GLBP, often less need for mHSRP yet there are still some situations where it's better than GLBP. (I recall mHSRP used to only be supported on the high end routers, but believe support has been extended to additional low end routers.)
With OER/PfR, which will dynamically redirect traffic on received gateway to another path, load balancing with mHSRP or GLBP can also be slightly better.
Without OER/PfR, and using a single gateway, but with peers, OSPF equal path costing might be better than gateway balancing, although perhaps a bit more difficult to configure. Same would be true for EIGRP unless you use unequal cost routing, and there are issues with that. BGP preference for single path, and conditions to take advantage of multiple peer routers also might be more troublesome than gateway load balancing. (For instance if you have two routers with complete Internet BGP route tables, that iBGP peer, and you only send data to one as a gateway, normally "equal" AS paths will use just the gateway router's external facing interface.)
PS:
As to finding hosts that are using the .4 gateway, besides sniffing, perhaps an ACL that matches against the virtual MAC and logs it, could reveal hosts configured to use it.
