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MP4 files are typically a combination of audio and video. Audio-only MP4 files are usually in the M4A format, which is a sub-format of MP4. M4A files are encoded with AAC (Advanced Audio Encoding) or ALAC (Apple Lossless Audio Coding). When converting WMA to MP4, you'll be converting to the M4A format. You may also want to consider the MP3 format, which is compatible with more devices. It will result in slightly lower quality than M4A. Adapter is a completely free conversion program that won't install any adware or unwanted toolbars. It is available for both Windows and Mac OS X. You can download Adapter from macroplant.com/adapter/. You can use adapter to convert WMA files to M4A or MP3 (as well as a variety of other formats).  There are lots of conversion services available online, but they require you to upload your files first and then download the converted files. This can take a long time with slow connections, and can eat up a lot of data on metered connections. Using a conversion program like Adapter will be much more efficient, especially if you have a lot of files to convert. During installation, you may be prompted to install ffmpeg. You'll need this for conversion, so follow the prompts to install it. The process is mostly hands-off. You'll be greeted with the minimalist Adapter interface. The songs will be added to the list of files to be converted. You can add all of your songs at once. This will allow you to select the output format for the conversion. These are both M4A formats, which are a subset of the MP4 format. Note that Apple Lossless Audio will result in a much larger file size than the original, while AAC will result in a file around the same size with minimal quality loss. If you want to play the songs on an older device, select "MP3 Audio". This will result in slightly lower quality but will compatible with nearly any device. By default, the converted files will be placed in the same location as the original files. You can change this by clicking the Gear button in the lower-right corner. Click the "Same as Source" drop-down menu at the top of the Directory section to choose a new location. You can have Adapter automatically import the converted songs into your iTunes library, or you can set a custom location. Each song win the list will be converted to the format you chose. The conversion process should only take a few seconds per song. Your converted music will be placed in the location that you specified. if you didn't change the location, you'll find the songs in the same location as the original copies. Now that the files have been converted, you can add them to iTunes on a Mac without any issues.

Summary:
Understand the MP4 format. Download and install Adapter. Open Adapter. Drag and drop all of the songs you want converted into the Adapter window. Click the drop-down menu at the bottom. Select "Audio" and then either "AAC Audio" or "Apple Lossless Audio". Click the Gear button if you want to change the output location. Click the "Convert" button to begin the conversion process. Find your newly converted songs.