do the 8 integrated interfaces on my FPR4120 support 1GB or just 10Gb
The datasheet confirms 8 x SFP+ (10Gb) on chassis
Those are SFP+ ports and as such support both 1 Gbps and 10 Gbps transceivers.
Note the overall spec for that model is 22 Gbps so obviously you won't magically get 80 Gbps if you fill it with 10 Gbps transceivers. :)