What computer operating system is your Premiere Elements 12 running on? And, have you updated 12 to 12.1 using the opened project's Help Menu/Update?
Please do so if you have not already.
Generalizing, you should be able to import 4K into Premiere Elements 12 and export it as 1080p.
The important consideration with "4K" is that it is a file size 3840 x 2160 pixels), and compression, file extension and other video properties are part of the overall consideration for 4K use in Premiere Elements. Assuming that there are no problems with Premiere Elements support....
a. You could use one of the NTSC/AVCHD/QFHD project presets ( 3840 x 2160 @ 23.976, 29.97, or 59.94 progressive frame rate)...not available under PAL.
b. You could consider an export such as AVCHD.mp4 1920 x 1080 @ 29.97 progressive frames per second.
Please review and give more details so that I can fine tune the details.