Which is better?
I am in love with the way Duolingo works, but I just don't know where I should go from here. I have a solid base on four languages (Spanish, French, German, Italian). Should I continue my tree with all of them together to find similarities and differences between them, or should I continue one tree at a time to avoid confusion?
one tree at a time that way you don't end up saying a four language sentence.
If you have no prior knowledge in a language I would definitely recommend not to do several at once. Otherwise, you can try. Four could be a little too much though...
Personally, I did do French and Italian at the same time. (I had learned both before.) But I usually did one week Italian, next one French. Then I decided to just concentrate on Italian and finish the tree. I'll probably make it golden before I move on to French... And refresh my Spanish because I'll go to Spain in a few weeks.
Try both ways and see which suits you best. I found simultaneous Portuguese Italian and French no problem, while also practicing German and attacking Dutch. If you already have a base in them, all the better: as you say, I found it great comparing them all at the same time. I also do Spanish->Portuguese and viceversa for this very reason and intend to do the same with French :-)