Can I modify code in older baseline and deliver to the stream

There is no way to "disturb" a baseline ... a baseline is immutable.   So in particular, you cannot make any changes to baseline 4.   What you can do is create a stream containing baseline 4, and then modify that stream.   You then ask about "delivering to the stream for that baseline".  I'm not sure what stream you mean by "the stream for that baseline" ... perhaps the "archive stream"?   If so, then yes, you can try to deliver that change set to the archive stream (although it might have a conflict, in which case you would need to resolve that conflict in a workspace before you can deliver the change set).