Hello Muhammad,
running four EIGRP processes and BGP is possible on a C3845. The load depends on the size of the each EIGRP database.
However, if you can describe the design requirements that led to this suggestion is possible to evaluate if four EIGRP processes are really needed.
I guess your network is divided into four parts and you don't want any part to know details (subnets) about each other.
If these four parts don't need to communicate I would suggest using 4 MPLS VRFs a single EIGRP process with four address-families one for each context. In this way you can achieve complete isolation.
Using four EIGRP processes provides routing isolation (no details) but doesn't provide forwarding isolation if default-routes are sent on each network part.
If the four parts need to communicate and the requirements is only to hide subnets details of other parts a single EIGRP process, (with configuration of manual summarization at the interfaces), is enough to reach this objective.
Hope to help
Giuseppe