Firstly to know about Mp4 and mV4 and their difference then we will discuss how to convert a m4v file to mp4 . you can look for some compressors and converters that can help you in doing every kind of conversion and compression, so that you may not waste your time, such as Uniconverter. 
M4v :
M4v is basically a file format was introduced by apple similar to mp4 and it is used to store video content in the iTunes stores. M4v is only used in Apple devices and it is not used in any other devices.
Mp4 :
Whereas Mp4 was introduced by Moving pictures expert group which is well known as (MPEG ) . Mp4 is a video container format that is basically used to store audio, video, subtitles, and images etc.
As compared to M4v, Mp4 is commonly used or supported in many different mobile devices and by many online streaming services.
Now we have to know that how to convert m4v to mp4. But before knowing about it first we have to know why we have to convert our m4v format video to mp4 format.
There is a lot of reason to convert m4v format video to mp4 but the most common reason is that to avoid the compatibility problems we have to convert our m4v format video to more device-friendly format.
We can convert our video format by using different converter which is available online.
Steps to convert m4v to mp4 :
As to convert our video format we have to follow the following rules carefully.
1: Install any video converter
Firstly in order to convert a video format we have to download any of video converter through history .
Then we have to run the video converter and then have to install that converter by following the guidance which will be seen on screen.
2 : Add the video you want to convert
Secondly, we have to add the video which we want to convert. In order to add files for conversion, we have to click on the add option which will be seen on our screen.
Then we have to click on the Add video button. Now our video which we have to convert can be seen on screen and some people are always confused that they can add only one video but now as the technology is grooming so all the video converters have an option that a user can use multiple videos in the same time. This is very helpful and now we can save our time.

3 : Specify the output file format
The most important step to convert video is to specify the output file format while using any video converter. In this step first, you have to click on the video tab to open the video tab and then you have to choose the output format. As in this case we are discussing to convert m4v to mp4 so we will select the mp4 format as our output version.
If a user wants to convert his or her video format to a particular device than have to click on the devices option.
As we discussed earlier that the technology is enhancing every day so it’s very easy to detect everything even nowadays mobile devices can easily detect faces so similar to this all the video converters have this option that we can automatically detect the model of any mobile. In order to detect automatically, we have to plug the device into your computer and then to click on the phone icon.
4: saving your data   
The last step to convert a video format is that all the available video converters saved the converted files in a folder by default and a user can easily change the folder.
If a user wants to change the folder there is an option to change or to create a new folder.
Now finally the last step is to convert the video format from m4v to mp4. All the video converter have the option of "Convert Video " by clicking on it our video will start to convert.
After converting our video will automatically open and we can easily transfer our video to any other devices like Mac, Pc, etc.
So these are the few steps by using which we can easily convert our m4v video format to mp4 video format . Similarly we can also convert mp4 to m4v format .