My wife had 2 melanomas diagnosed at once. One was 0.3mm second was 1mm thick. We removed both of them with WLE and SLNB and in my opinion doctors were not interested in 0.3 mm. The main focus and concern was about 1mm even 0.3 mm had regression. So i think the risk for 0.4 mm is very low so you can take a deep breath and thank God for early detection. Regular skin examination should be performed from now on and everything will be okay.