I'm using Premier Elements 15 with footage from two Panasonic 4K cameras. I don't have your problem. Perhaps you can go through your steps and I may be able to help.
I could upload all 4k footage and cut it. When I want to save it on my computer I chose 264, 4k and demand vor a 5.16 Gb HD Version before I render and save it. But after Export the film has just 68 MB on my computer. I can not screen it in this size! I retried it several times but nothing changed. I never had this downsize Problems with Elements 11.
You are not offering to supply the steps you are taking, so there is little room to guess why you are getting what you are getting.
Export&Share > Devices > Custom > Advanced Settings (button) > Match Source (button).
Then check the bit rate settings. They should be about the same as your source.