The restriction of interfaces is based on software. What I have seen is that Pix will not recognise the interfaces beyond the software limits of the model. For ex. PIX 525 can detect 8 interfaces. But the additional interfaces may be in disabled mode. With Restricted licenses, even if additional cards are installed, it will show only the interfaces within the software limit. But you upgrade to unrestricted and it detects the additonal interfaces. But what I believe is that you may be able to install 2 4-port cards and it may detect only 8 interfaces and rest will be lying dormant until there is a new software version which may be able to support additional interfaces. This is purely my thinking, do try it with caution.
This link provides the circuit board options possible with PIX 525, which does not cater for your option.
This has come up in the past and I can confirm that what the previous poster answered is indeed true. The bottom two interfaces on the 4FE in the right most slot will be disabled. But you can install 2 4FE cards and a VAC+ in a 525 to achieve the 8 interface maximum. I know this is not ideal but future versions of PIX code may increase the max interface limit and you will then be able to leverage the additional two interfaces on the second 4FE. Good luck!
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The following licenses have been activated on my router :