Solved by KFrank in post #2 
              
              
                Hello Alex! 

You say in your title “multiclass+multilabel,” but you say in your post 
itself “where each pixel may be the one of [0,1,2,3,4].” 
To clarify the terminology: 
Yes, I would call this “multiclass” because you have five different 
classes (one background plus four non-background).  (If y…