Exporting in 3840x2160

You can find After Effects in the Creative Cloud Desktop application under the "Apps" tab.  It can be located a little more quickly by selecting "Video" under the Catagories section at the left.  It's grouped with Premiere Pro, Audition, Character Animator, and Media Encoder.  After Effects can be installed or opened from there.  If it's not included with your Creative Cloud subscription, it should run in trial mode.

Detail-preserving Upscale is currently the best algorithm we have for upconverting video within the Adobe video and audio applications.  It's meant to produce good results for one upconversion level, like SD to HD or HD to UHD, and does better with full resolution source footage formats that are good for editing (like ProRes and DNxHD) than highly compressed formats that are good for delivery (like HEVC H265 or H264).

A common workflow is to use the 1920-by-1080 clips in a 3840-by-2160 Sequence with your 3840-by-2160 clips during your rough cut, using "Set to Frame Size" to change the Scale property for the 1080 clips from 100 to 200.   In this workflow, do not use "Scale to Frame Size" as that leaves the Scale parameter for the 1080 clips at 100.  When you lock the picture (you're not making changes to the assembly) or are doing your fine cut (you will make minimal changes to your assembly), use Premiere Pro's "Replace with After Effects Composition" option to send the 1080 clips to a 3840-by-2160 Compositions in After Effects.  The first time that you do this, you'll have to save an After Effects project (recommended: save the After Effects project file to the same location as your Premiere Pro project file).  In After Effects, change the Scale property back to 100 then apply Detail-preserving Upscale (Effect > Distort > Detail-preserving Upscale), clicking the "Fit to Comp Width" button in the Effect Controls.  Choose File > Save in After Effects and then jump back to Premiere Pro.  You will probably need to render the After Effects Compositions in your Sequence for smooth playback.  Optional: In this workflow, you're not likely to return to After Effects to make additional edits to the Composition.  As such, you can right-click the Comp in the Premiere Pro Sequence and use "Render and Replace..." instead.

As a rule of thumb, if I'm scaling clips up to 125%, I usually use the Premiere Pro Scale property.  125% to 150% is where a take a closer look at whether or not the footage is holding up well enough on the Premiere Pro side.  If the scale change is 150% to 200%, then I almost always use After Effects.  For more than 200%, I consider using Topaz Labs Video Enhance AI.  I also keep an eye on what's happening with upconversion in Photoshop, but this response is already getting pretty long so I'll stop here.