Your first 5-4 observation - don't take this the wrong way; but you are now with the Big Boys.....
CFA assessment are there to develop you and promote you. at L4 you are in competition with other referees, so there is a standard expected by the players, club officials and the observers. Too many new L4 think they are the "finished product" because they have made L4l; but they are way off. The first observation can be a shock to the system. I had one who had been quickly promoted to L4 a few years ago, and he had a poor match and got a suitable report. But he was shocked on the following - a player already had a YC and handled in the area (near the penalty spot), goalkeeper on his line so in my view he would have saved the shot. But the referee sent him of for a DOSGO - H; whereas the correct decision was a 2nd YC and RC. He could not accept that he was wrong in law and kept saying it was the same outcome, the player was dismissed - But no, he was wrong in law and this was a KMI so a 6.5 had to be awarded..
This may sound tough, L4 observers are their to find the best referee, not smooth your ego.
But, these harsh early observation work, as you stop doing the bad things and become better referees. I hope you took the advice from the observer on how to develop, and I think one of those would have been stronger on the moaning / dissent to you.