SNAT pool Inet port exhaustion

An IP address can have roughly 65K connections. If you are hitting limits and seeing log messages regarding port exhaustion, then I would suggest to check the persistence settings on the VIP as well as the load balancing method. Technically, you have 4 backend servers (65K connections per self IP to each backend server), then you have 15 SNAT IPs (15 x 4 x 65K...should be a lot). Assuming good load balancing and persistence settings, you should see even traffic distribution. Also check idle timeout settings and long-lived connections to make sure you're nuking clients that are inactive.

Also, you mention SNAT pool. Make sure the VIP is using the SNAT pool in order to properly use all 15 IPs.