You’ll need to get an ISL license in order to connect the switches together. You don’t want to connect all four switches. You’ll want to connect switch1 from SAN1 to switch1 of SAN2 and switch2 from SAN1 to switch2 of SAN2. When you do this you’ll need to extend the fabric from switch1 of SAN1 so that it covers both switches. The same goes for switch2 of SAN1.
If you do this carefully and correctly you won’t notice any impact to the existing setup. If done wrong some or all of your servers could loose their connectivity to the SAN.
If these are older switches you may need two cables between each pair of switches in order to have a full speed connection between the switches.