Hi,
If you need to have Router-B behind the SSG with a puclic IP, you will end up using one more public IP, because int-4 needs an IP from the same 188.x.x.x subnet.
The easiest way to get this done would be by configurign a prvate IP on R-B and mapping it to MIP->188.x.x.235 on int1. If this is an IPSec VPN, please ensure both VPN endpoint and R-B support NAT-T.
If you do not want NAT, you can follow the suggestion provided by Nikolay.