Plug ur N4 to your computer. Fin the "ringtones" folder, and drag the preffered file there. Unplug device. Go to setting, sound, ringtones, and select the file you've wanted.
I have an app called Player Pro that I use to play music and podcasts. One of the menu items on it is to use a song as a ringtone. Import a song to the phone ( I use Tunesync to go from iTunes to the phone) and then just bring up the song and set it to ringtone. Takes about 2 minutes to do the entire process.
I used an app called RingDroid back when I had the OG Evo, and have just been porting them over since then. It's free and allows you to snip out whatever section of the song you want. Think I used Dropbox to get them on my Nexus 4.
I saw that there are apps to make your own ringtone from your mp3s. However, I also saw plenty of freebie downloads of songs already done. I wanted one that I could hear and would not be confused with other phones so that I would know mine was ringing. I chose the patriotic category and am using the Washington Post March. I'm pretty sure I'll have a unique ringtone for the duration