generally, secondary fermentation is recommended in a carboy or something with similar low-volume headspace. as fermentation slows down, the danger of air exposure increases.
3 inches of topping off into a bucket is an awful lot of topping off... but i am a little confused from your description..
did you begin primary fermentation in a bucket and then rack into another bucket according to the SG given in the kit instructions?
have you been fermenting for a total of 4 days or just 4 days since racking into the secondary bucket? how long was the primary ferment and what was the SG when you racked into secondary?
bottomline, i think it would be wise to get into a carboy or betterbottle as soon as you can and then follow the instructions based on SG readings, not the arbitrary time/calendar the instructions estimate as rarely does a complete ferment follow the calendar timeline exactly. so many factors can affect the time.