You should always wear aligners in matched pairs. The trays work in concert with each other.
The best option is to get a replacement aligner to make sure that the teeth move properly. If this is not available to you then the next best solution is to switch to the next lower aligner and wear it for a longer period of time. So if you lost Aligner #1 on day 4, you would switch to Aligner #2 lower and it for 10 days. At the 2 week point, you would change the upper aligner to #2 but continue to wear the same lower aligner (#2) so that the numbers on the aligners match.
Lost Invisalign Tray - Orthodontist
I would contact your orthodontist as they can order a replacement tray for you through Invisalign. In order to decide if you should move up or down in tray numbers, an orthodontist should examine you wearing the trays to see how well they fit. Good luck!