The 4200 series MCUs went end of sale in September 2013, so there is no way to order parts or options for it any more, and the device is also past its end of support date of 30 September 2018, so is also now unsupported.
The 4210 had a maximum of 20 ports for Continuous Presence Video Participants - if you needed 40 ports, the 4220 was the model you needed, you can't do it on a 4210.
I'd highly recommend that you consider looking for a suitable replacement option that is current and supportable (such as the Cisco Meeting Server)
Wayne
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