Weight is an attribute that is local to each individual router. There is nothing that prevents you from using weight on both R3 and R4 to direct traffic to the ISP of your choice.
For your new topology now, remember, you have full control of your outbound traffic. Just like you did with your topology with a single router on the company network, you simply configure the same thing on each of the R3 and R4 routers. Set up the prefixes you want to send via ISP1 and those you want to send via ISP2.
Now in order to configure R3 as your primary router and R4 as the backup router for your traffic, you can do this in multiple ways. These include:
- Configure HSRP, VRRP, or GLBP which are gateway redundancy protocols, on the enterprise-network-facing interfaces of the routers and make R3 the primary gateway and R4 the secondary/backup.
- Configure routing (either IGP or iBGP) between the edge routers and internal routers on the enterprise network to route traffic via R3, and have backup routes go via R4.
- You can even configure load balancing/sharing using GLBP, or equal cost load balancing of a routing protocol to send traffic to both R3 and R4, taking advantage of the bandwidth available for both devices to each ISP.
A couple of notes:
- If internal traffic is sent primarily to R3 using one of the above methods, then BGP routing in R3 will take care of where to send such outbound traffic, ISP1 or ISP2, depending on the destination. The same goes for traffic to R4.
- The benefit that iBGP between R3 and R4 will provide is if the physical link between R3 and R2 fails, for example, then traffic that hits R3 that should be routed via ISP1, will be sent to R4, and then to ISP1. In case of such a failure, it may take BGP some minutes to reconverge, so you may need to set up BGP Additional Paths in order to allow for R3 to have the additional path added via R3 using iBGP.
I hope this has been helpful!