ReadyNAS 4200 + 10GBe Option

1) Are these CX-4 ports in addition to the 1GB onboard NICs

Yes. Both RN12G1220 and RN12G0620 support 2xGigabit ports + 2x10GbE ports (CX-4)

2) All of Netgear's switches that support 10GBe only have XFP Fibre modules, I can find CX-4 modules for say the GSM7328FS so how can I connect my ReadyNAS to it at 10GB or do I need to go somewhere else for a switch.

4200 also supports SFP+ 10GbE connection. The Netgear PN for that is RN12S1220 and RN12S0620. Similarly those units have 2xGigabit ports + 2x10GbE ports(SFP+ , fiber or copper).

NETGEAR ProSafe GSM7328S-200 (or GSM7352S-200) support the CX-4 10GbE ports. 2 switches needed for a stacking configuration with AX742 24Gbps Stacking Kit.

3) The ReadyNAS 4200 seems to only come with the 10GB modules or "with a 10GBe option" does this mean I can add CX-4 ports later on if I decide to buy a 4200 without the 10GBe adapter.

4200 comes with 3 group of configurations:
1) with standard gigabit ports but can add the 10GbE package later (CX-4 or SFP+)
RN12T1210
RN12T1220
add-on cards -
RN12T2SFP (2x 10GbE ports SFP+)
RN12T2CX4 (2x 10GbE ports SFP+)
2) with 2 gigabit ports + 2 10GbE ports (CX-4) built-in
RN12G1220
RN12G0620
3) with 2 gigabit ports + 2 10GbE ports (SFP+) built-in
RN12S1220
RN12S0620

Hope this helps.

Boba