What WMV to MP4 actually writes
WMV is a legacy Microsoft format. Phones and current websites refuse it.
The file is re-encoded to MP4 on your device. Some old WMV codecs cannot be read here, and then the conversion fails.
Quality and the WMV file you should keep
If it fails, open the WMV in the Windows app that still plays it and export MP4 from there. A successful MP4 will not look better than the WMV.
A related step, if you still need one, is MP4 to WMV.
Why WMV to MP4 is its own job
WMV to MP4 sits on this site because WMV and MP4 are not interchangeable, even when they look similar in a file list. WMV is a legacy Microsoft format. Phones and current websites refuse it. Old Windows Media files are converted to MP4 so they play anywhere else. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4.
The file is re-encoded to MP4 on your device. Some old WMV codecs cannot be read here, and then the conversion fails.
The WMV file you already have
WMV is a legacy Microsoft video format. Modern phones often refuse it. Some old WMV codecs cannot be read in the browser, and the conversion then fails.
Keep the WMV until the MP4 has picture and sound.
What a MP4 result is for
The download has to be MP4, because that is the format the next step named. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4. If the other person can open only one video format, MP4 is that format.
A new encode cannot look cleaner than the MP4 you started with. A remux on a desktop is the lossless path when the streams are already compatible.
How to tell that WMV to MP4 worked
WMV to MP4 is worth using only when MP4 is what the next step actually named. WMV is a legacy Microsoft format. Phones and current websites refuse it. What you get from WMV to MP4: The file is re-encoded to MP4 on your device. Some old WMV codecs cannot be read here, and then the conversion fails. Before you trust it: If it fails, open the WMV in the Windows app that still plays it and export MP4 from there. A successful MP4 will not look better than the WMV. The file stays on your device. The first run downloads a converter, so keep this tab open. A phone runs out of memory on long files. A computer is the safer place for a big clip. A second pass rarely helps. If the first MP4 file is rejected, read the error from the program that rejected it. Another conversion of a bad result usually makes the file worse, not more compatible. When you are done, you should still have the WMV original and one MP4 download that you have opened. If those two are the same kind of failure, this was the wrong tool and the page above is the place that says so. Video files hide the real problem inside the container. WMV to MP4 re-encodes on your device. It does not silently copy every audio language and every subtitle the way a desktop remux can. If you needed those extra tracks, this download is the wrong result even when the picture plays. Convert a one-minute piece before a full movie, and stay on the tab while the converter downloads the first time.
Here is the decision for WMV to MP4. You start with WMV and you stop when a real program accepts MP4. When FFmpeg can read the WMV, you get a normal MP4. Soft subtitles such as ASS or PGS, and extra audio languages, are awkward or dropped when a tool re-encodes. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
If it fails, open the WMV in the Windows app that still plays it and export MP4 from there. A successful MP4 will not look better than the WMV.
A related step, if you still need one, is MP4 to WMV.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.