It depends a bit on what you're working on, for example if you're trying to develop more leg speed during hard efforts then it could make sense to do some of the 'on' intervals in a moderate gear but at higher leg speeds but in general the choice of gear isn't really the important thing. The most important thing to pay attention to during intervals is the overall intensity. Whether you measure that with HR, power or just pay attention to how hard it feels (RPE, or Rate of Perceived Exertion) the key thing is to make the 'on' periods sufficiently intense and the rest intervals sufficiently easy so that you'll get some recovery before your next effort.
From that standpoint your first example 'on' at 120 rpm in a given gear, rest at 90 rpm in the same gear probably won't generally be either intense enough for a 4 minute effort or conversely the rest interval would probably be too difficult. If you want to emphasize leg speed (which isn't always necessary but can be useful for certain events like crits) then you could still go 'on' at 120 but you could also shift as necessary to make sure the 'on' interval works you very hard for the duration but the rest interval allows sufficient recovery to complete the entire set of intervals.
A 4 minute interval targets VO2 Max or power at VO2 Max so the keys from an RPE standpoint are that if you pace it evenly the first minute and a half or so shouldn't feel too bad but by minute two you should be at or very near maximal breathing and the final two minutes should be very hard with ragged maximal breathing and a lot of effort. It should be a bit easier than your best possible single effort for 4 minutes but only enough that after recovery you can do another, and another and finish a set of 5 or 6 total. If you're doing them much easier then you're not really targeting VO2 Max work and should be doing longer efforts in the 12 to 20 minute range to target Threshold fitness. If you can't complete the 4 minutes or can only do one or two reps before backing down the intensity then you're pushing a bit too hard.
Anyway, the key thing to manage is intensity appropriate for the duration of the interval which more or less defines the physiological system you're targeting. What gear and cadence choices you use are a secondary consideration and not that important unless you have a specific reason to work some leg speed into your intervals.
Good luck,
-Dave
