Is it better to submit a paper to a journal "normal" issue, or wait 4 months and send it to a more focused "special section" of the same journal?
I would strongly recommend choosing the special issue unless you are studying something time sensitive due to competition (e.g., you have a known competitor who might publish tomorrow) or emergence (e.g. an ongoing natural disaster).

First, depending on how the journal works, you might not actually have any delay in publication.  Check and see whether the journal has a "just accepted" or "online first" section.  If so, then your article will not have to wait for a special issue package to become available and citable.  Instead, special issue articles will become available asynchronously as they are processed, and then packaged later.

Second, special issues, while they are reviewed no less tightly, have a specified audience that improves your chance of acceptance.  You can much more readily pass the "audience interest" bar that many high-profile journals apply, since somebody else has already made the case that work in this area is of broad interest. Also, because reviewers are picked with expertise in the subject, you are less likely to get a bad match with a reviewer who knows little of your subject.

Third, you are likely to have a higher long-term impact from the publication, as it is grouped in a package with others like it, where it will be easier for people to discover or seek it out.

Finally, in most fields, four months is just not a long time.  Finish your paper now, send it to the journal special issue before the deadline, and move on with your work.