You can top dress a plant with supersoil, its actually encouraged IF.....A. the plant needs it.. Are you seeing any deficiencies in the leaves at all? If not then theres no reason to give the plant more food unless you anticipate the plant needing it.... . And B. where is the plant in its flowering cycle, I would NOT top dress a plant after 4 weeks running a 8-9 week strain.... The nutrition will come through too late and your plants will take forever to finish and they'll finish green green most likely...
Supersoil is highly amended soil, so theres nothing wrong with using it as a top dress... Or you can top dress as the above posters mentioned... its all about what your plant needs and where in the flowering cycle your plant is #1, and #2 what you have on hand... If your pushing into week 5-6 and you start noticing deficiencies... you can use a tea or hit them with some molasses and try to limp through like that... But after the 2/3 to 3/4 point of flowering in organic soil/water only growing, you let it ride... Deficiencies are normal at that point on and are a good thing for the most part... It means your close to getting it right... Getting an 8 week strain to start fading (showing deficiencies) at week 6 is nailing it pretty much...
But to your main question, does your plant need a top dressing? Is there anything in the leaves that would make you think it needs more food? If yes and your pre-week 4... Go for it... But I would no fill it... maybe 1 to 2 inches of SS on top thats about the most I would put and see how they finish... If you see they finish perfect... next time add that extra inch to the pot pre-flowering them...