I believe historically Nexus devices have received 2 OS updates in their life cycle, right?
Nexus One - shipped with Eclaire then got Froyo and ended on Gingerbread
Nexus S - shipped with Gingerbread then got ICS and (likely) ends on Jelly Bean
Galaxy Nexus - shipped with ICS then got Jelly Bean and (likely) ends on Key Lime Pie
I highly doubt the NS will get anything past JB (at least officially) considering its aged hardware.
As for the Galaxy Nexus, personally I think it will depend on the hardware of the Nexus 4 - if it's a quad-core then the GNex will likely stop at Key Lime Pie, but if the Nexus 4 is only a dual-core (which I doubt) then the GNex has a much better chance of getting an update past Key Lime Pie.