You can start right here and review the models that supports SFPs. FWIW, your not going to find a low or med grade device that does this or nothing cheap.
http://www.fortinet.com/products/fortigate/index.html#mid
Once again, your adding more complexity into your setup and it' s not clear as to what/why you think you need to terminate the floors on a dedicate fiber ports on a L3 firewall device.
And if you want to apply a 2nd unit of have any HA concerns, than you boxed yourself into a tight box if the fiber are connected directly to the fortigate.
And yes you can apply a L3 ip_addressing on a fiber interface & in the same fashion as copper port.
See the attach diagram of a smarter solution using the current but with a FGT in the mix. With is design, you can terminate each floor on a vlan unique and on the FGT
i.e vlan 101 floor1
vlan 102 floor2
vlan 103 floor3
vlan 104 floor4
This would route all traffic between floors at the firewall. if you don' t need that level of policies, just terminate the l3 SVI on distribution switch and set the default gateway to the FGT.
And if you install a stackabke model of switch like the 2960 EX2/4K, etc... you can scale up each floor as you need access-ports.
If you later need more firewall , you just add them to the distribution switch. If you need more bandwidth, you enable 803.ad LACP bundles between firewalls.
Keep It Simple & Straight aka KISS