have both premier 4K 1080 and express 4K1080, can't see any difference between the two,only that on the express the box has a faint blue light when connected,the premier doesn't have a light on the box so unsure that it's connected until you try the remote,
There aren't really any devices with those names. So, I'm going to guess which device you actually have, and try to answer your question.
The Roku Premiere model 4620 is a 4K device (all 4K devices also support 1080 and 720) that was in production from October 2016 to September 2018. It looks like a Roku Ultra, both having the Roku 4 form factor. It was dual band (2.4 GHz and 5 GHz). There is also a + version of this device.
The Roku Premiere model 3920 is a 4K device in production from September 2018 to May 2021. It looks like a Roku Express. It is 2.4 GHz only. There is also a + version of this device.
The Roku Express 4K model 3940 is a current device since May 2021 and is only sold at Walmart. This is a dual-band device. There is a + version of this that is widely sold.
Does that help? Or is there a more detailed question?
You can check the model number at Settings > System > About
