The status change from "under review" to "reviewers assigned" indicates that the editor has decided to send your paper to another reviewer. There could be several reasons for this. The reviewer might have realized that he/she doesn't have the time to review the manuscript or because he/she feels that there could be a potential conflict of interest. another reason could be that after going through the paper, the reviewer has realized that he/she does not really have the required expertise on some aspects of the paper.
The status date changing with the status remaining the same indicates that the Editor has accessed the database to check your manuscript. This could be to check whether any of the reviews have come in.
Since it has been 4 months from submission, you should definitely send an inquiry. If you have the editor's contact details, you should write directly to the editor. If the journal website does not provide the editor's email id, you should write to the journal office. Some journals allow authors to send inquiries only through the online submission system. If that is the case with your journal, you should send an inquiry through the submission system.