First off, the MetroPCS S4 is the M919N model number. The M919 is the Tmobile variant.
You'll notice that there isn't ANY development at all for the M919N MetroPCS S4. That is because the M919N from MetroPCS is identical to the M919 Tmobile variant (they are both on the same Tmobile network too) and a majority of the roms and mods (including root methods and recoveries) that are made for the M919 will work for our M919N without any patches or further modification. In fact EVERY rom and recovery posted in this (MetroPCS S4) forum is actually for the Tmobile S4 M919 (except for the two out of date roms that I made and posted a while back).
As far as I'm aware, Cyanogen has an official rom for the Tmobile M919N and it has nighties and updates, I don't know if it is 4.4.4 specifically (4.4.4 may still be a beta). The rom you mention is not officially from the Cyanogen team and that's why it wont get nighties. You'll need to change the device model from M919N to M919 in order for the Cyanogen installer to detect your phone as the Tmobile S4 and install Cyanogen. Or just flash a Tmobile S4 stock rom (like Insane KitKat for instance) and that will change the model number to M919 and the Cyanogen installer should work then.
There's really no point for devs to develop for both of these S4 variants when the Tmobile S4 roms will work for both.
Lastly, we don't have a dev that has our M919N like we had with the MetroPCS S3. The MetroPCS S3 wasn't identical to any of the other S3's and so to get ANY development for that phone, a few of us donated to Shabbypenguin so that he could get it and when he got it, he did a lot to make that phone as good as it was. He's the MAN!