Quattro Stagioni pizza comes out unbalanced
Seasoning is definitely a big part of your solution. Look at this recipe from Wolfgang Puck. 


  saute the mushrooms in the olive oil until done. Season with salt and pepper


and


  Sprinkle basil over tomato section and grated Parmesan over the mushroom section


Taste your mushrooms and artichokes with different cheeses, herbs, spices, whatever. Certainly use additional salt if it seems warranted. Most importantly, taste your toppings with various complimentary flavors, always keeping in mind that you want the pizza to be a harmonious whole. Experiment!