As the error message explains your first conv layer uses a different number if in_channels than the one stored in the pretrained state_dict (4 vs. 3).
To fix this issue you could

remove the alpha channel from your images in case it’s all 255 anyway
load the original model with 3 input channels and replace the first conv layer
do as described before, but fill the first 3 channels with the pretrained weights and leave the last channel randomly initialized (not sure if that’ll work out)