Welcome to the forum.
Unfortunately, you have posted to the Tips & Tricks sub-forum, which is a repository for articles on how to do things in Premiere.
I will Move your post to the main Premiere Elements Forum, where more will see it.
[Moved to Premiere Elements Forum]
Meantime, try using Publish & Share/Computer/AVCHD with the YouTube HD preset.
To get off on the right foot with this, what are the properties of this .mp4 source that you are bringing into a Premiere Elements 12 project?
What computer operating system are you running Premiere Elements 12 on?
I am looking for video and audio compression, frame size, frame rate, interlaced or progressive, pixel aspect ratio, duration for starters.
Based on these properties, what have you or the program set as the project preset for the project?
Based on the information that you supply, I will comment on the YouTube choices I would make for the best quality result for the upload to YouTube. What is your intent:
a. export your Timeline to a file saved to computer hard drive and then upload that file to YouTube at the YouTube web site
b. upload your Premiere Elements 12 Timeline to YouTube via Premiere Elements' Publish+Share/Social Websites/YouTube?
Your choices may be forced (limited) by the Premiere Elements....Social Website/YouTube limitations of 2.00 GB/15 min.
Looking forward to your follow up.