Hi,
It is always better to get the pre cut adhesive, as this allows it to be put on much easier.
Type in something along the lines of "LG G4 LCD Adheshive"
Now, the reason why your screen is more than likely lifting is because you haven't prepped the bonding area correctly, the frame and the LCD need to be prepped correctly to get maximum adhesion.
For this, you need to clean any old adhesive that is on the frame, and any fingerprints, grease, dirt, oil, dust etc. From both the frame and the LCD, use a small cloth and 99.9% isopropyl alcohol to clean the old adhesive off and any dirt etc. You need to make sure it's extremely clean to get a good bond.
After you have cleaned the frame thoroughly, you can then clean the edges of the LCD with the isopropyl alcohol.
After its all clean, apply the adhesive, and push it down, trying to not get any air bubbles underneath it, and then apply the new LCD, and apply pressure to that, if you can, use clamps to hold down the LCD for a few hours whilst it gets maximum adhesion, I would suggest a minimum of 12 hours.
If you have any questions let me know. :-)