Not sure if you still need help with this but I would like to ad my experience in hopes it’ll help.
I have NEVER gotten unreal 4 to successfully sign an apk for me. Not sure what i’m doing wrong but I ended up doing it manually which takes me about a minute.
package for android (I use etc1 for max compatibility).
Once its made I move it to my jdk bin folder (which is C:\NVPACK\jdk1.6.0_45\bin on my computer).
Once its there I press shift+right click and open a cmd window.
Once I have that open, I simply follow the steps at the bottom of the page here: http://developer.android.com/tools/publishing/app-signing.html under Signing your APP Manually
Don’t forget to also zipalign your apk.
Ive done this multiple times for our game Lux Protocol, which is now on google play. Hopefully it helps as this took me forever to figure out.