I submitted my manuscript to a journal (Q4), according to SCIMAGO ranking and unfortunately it got published. I say "Unfortunately" because I think it should be submitted to a better journal. Now, is there any way to withdraw this publication? According to my googling, papers published in Q4 journals do not have any weight in CVs for academic positions such as postdocs.
I would not worry too much for serveral reasons.
It is probably early work because otherwise you would have known.
Second, many great authors started their academic career in low-rankend journals. Good publications maintain their value over the years.
Third: search engines do not differentiate between journals.
Fourth: I judge the quality of work on the work itself, not on the journal it was published in.
Fifth: I have recently reviewed a publication for a low-ranked journal and seen the other reviewers’ comments. This was just a professional review that would not be different for a higher ranked journal.
Finally, the ranking of a journal is not a constant and will rise if the journal succeeds in attracting good publications.
No, you cannot withdraw a paper after it's been published to submit it elsewhere. In fact almost all journals will have a "submitting a paper here indicates that it has not been published elsewhere" policy. For example in Springer's author ethics page,
The manuscript has not been published previously (partly or in full), unless the new work concerns an expansion of previous work (please provide transparency on the re-use of material to avoid the hint of text-recycling (‘self-plagiarism’)).
Further, since the journal you submitted to has committed time and resources to peer review + publishing it, withdrawing it now is very unfair to them.
That's not to say you can't pretend that you've discovered a critical error and need to withdraw the article, and submit it elsewhere afterwards. But if you are discovered, you can expect the new journal to retract your article, and to be blacklisted by both publishers.