EIGRP is able to load balance traffic across multiple paths, so yes, the traffic would be balanced across both routes. BUT, it would be balanced proportionately. In other words, if the metric for the first route is, say 100, and the metric for the second route is 20, then the ratio between those two metrics is 100:20, or 5:1. This means that the second route will carry five times as much traffic as the first route. (Remember, a higher metric or cost is a worse route).
The amount of traffic each route carries is proportional to the metric or cost of each route. The important thing to note here is that EIGRP is able to achieve this, while other routing protocols, such as OSPF does equal cost load balancing, regardless of the metric of each participating route.
In your sample case, both successor (R3) and feasible successor (R4) have been defined. What are we going to achieve with ‘variance’ command to load balancing between feasible successor ?
R4 AD is lower than R3 FD, so R4 will become feasible successor and add it to the routing table.
The variance command is used to tell a router if it should load-balance traffic across the successor and feasible successor. By default, in an unequal cost scenario, only the successor will be used to forward traffic.
By issuing the variance command, we are not changing the status of the FS or the successor. However, we are telling the router to add the FS to the routing table (assuming the variance command includes the appropriate ratio as described in the lesson). Thus, load balancing, based on the variance multiplier, will take place between those two paths.
This is an issue of terminology. A successor is defined as the best path to a particular destination. The only way you can have two successors is if they have exactly the same metric. So by definition, if you have two EIGRP successors, then equal cost load balancing will be performed between them.
Unequal traffic will only take place between a successor and a feasible successor, assuming the variance command is configured appropriately as in the lesson.
Now if you want to force traffic across two successors in such a way that one successor takes more traffic than the other, then you will have to modify the metric of one of those paths. This will make one of the successors a feasible successor, and thus you can use the variance command to perform unequal cost load balancing. You can adjust the metric by modifying the bandwidth, load, or delay on an interface which will change the metric accordingly.