If you want to max out yields, you need 1000PPF of light power hitting all areas of the canopy. In a 4x4 tent this will require around 1000 watts of quantum board lighting spread evenly around, or 1500-2000w of CMH/HPS /Blue-Red LED's
Your 2 quantum boards will produce 200 watts of power, which is equivalent to a 400 watt HPS. They will produce plenty of yield, up to 400 grams in your 4x4 tent. With that
Viparspectra R900 (actual power draw 430w) my best yield was 320 grams (0,75g/w) using it solo in a 4x4 tent.
You can always get more lights and improve your yield, the real question is; is the investment worth it for you for the increased yield? Your 2x100w Quantum Boards will do more then fine to give you an amazing yield in a 4x4 tent, that is high quality.
Remember that 400 grams of yield for your quantum boards is the upper limit, there is no guarantee you will get there on the 1st try, I would be looking at a 320-350 gram yield for your first 1-2 tries. If you feel this is yield is not enough, you need to get more lamps.
You will however have a healthy garden and canopy full of buds in that 4x4 space with those two quantum boards.