Going from 2.9Wkg to 4 would be quite a goal to reach, certainty achievable over a few years.
Number wise I guess that 4W/kg could potentially place you well up there in Expert class races, depending on your technique.
Along with your FTP value you most likely got HR values as well, whilst not as good as power, training with a HR monitor, even on a commute can increase your FTP.
Things like 2 * 20 mins, 4 * 15 or the like are classic sessions for this type of training goal, and potentially doable on a commute. Get yourself a HR monitor and train at those levels that you saw in your test.