Problem:
Right now we have to go to the market to check to see whether or not we have all four boosters active. That's an unnecessary step.
Solution:
Please show what boosters we have active on the arena / boss battle / PvP pages, maybe between the team pics and the battle stats. Clicking on an empty booster slot should take us to the booster page of the market where we can activate (or purchase and activate) an additional booster. Thanks!