[Xperia Z4 Tablet Q&A, Help & Troubleshooting] allofmex:
Since my battery became almost unusable, I took the risk to replace battery myself.

And it was successfull! :o

Here some hints:

First of all: Make sure you have good tools, mainly:

Only use a very thin and a bit flexible opening tool (I used iPlastix tool, the plastic version)

Use a good heatgun with small opening adapter (< 1cm). Make sure the heatgun can be set to low temperatures, I strongly suggest one that can be set to exact values (In my case "Steinel HL 2020E", can be configured from 80 degree upwards in 10 degree steps)

Build yourself some kind of stand for your heatgun that you can heat the display while having both hands free (I used a small camera tripod and tape)

If you do not have ordered the new adhesive yet, use a internet photo of it to see where the adhesive is located and how deep it reaches. (the edge where adhesive does not reaches edge is headphone jack). Choose a spot with thin adhesive (not an edge) to start.

Heat up this spot (in my case: heatgun on 90 degree celsius, 5 cm distance from outlet to surface) and while still heating, carefully insert opening tool. Do not use force, heat longer if needed. Than slowly move around the tab by heating and dragging your tool. (here the mentioned stand for heatgun helps a lot). After you opened some way, insert a thin piece of platic to prevent adhesive from reattaching.

Again: NO FORCE. When old adhesive is warm enough, you can easily "cut" it with your tool. If you still feel resistance, you need to heat more. (on spots with larger adhesive areas I switched to 100 degree). When you are around, the display should be able to lift, if not, heat again remaining spots and use opening tool until screen can easily be detached. (never bend/pull) There is not adhesive in middle of display, only at corners as you see on new adhesive.

The opening itself was surprisingly easy. More annoying was to remove the old adhesive remainings. I tried isopropylalkohol, but it did not help much. Most of it was removed with some plastic blade, mooving/scratching/pusching permanently against the end of the adhesive trail. It takes time, but it removes it in 0.1 mm steps (maybe someone has a better idea).

Make sure that you work on good mat/base that you can put force without bending screen. I had some sticky but flat remainings on display surface, but this should not be problem.

Removing battery adhesive: use colored edge of pull tab ONLY to grab it. As soon as the white part gets visible drag always on the part that is just coming out behind battery. The first tab teared for me because I did it wrong and it was difficult to remove the remaining. The second tab could be removed completely with ease. (How to detach the cabling, see the youtube teardown videos)

At this step I suggest to replace the main camera lense if the lense coating is broken (as usually on Sony :rolleyes: ). Do not remove lense from inside, only the round part gets replaced. Heat up lense and push from inside outwards! (see attached photo)

Replace pull tabs and battery, reconnect cable and TEST tablet before attaching new adhesive! (you may use sony test menu by calling *#*#7378423#*#* in phone app)

Another tricky part is to apply the new adhesive. You have only one chance, as soon as the adhessive touches a surface it sticks, if you try to detach it again it will stretch and stay stretched, so better don't try. Seems the adhesive has to be applied to display first, so detach cable of display, test carefully how to hold and handle before removing plastic covers of adhesive and then: good luck... (Result was "almost" ok in my case. If result is tolerate able, keep it that way. Trying to change again will just worsen the result. Cut parts sticking out display edges)

Reconnect remaining display cable, test functionality again and glue the other side.

Congratulations, you got 2-3 years more of tablet lifetime. Lets hope the guys at Sony get it done to bring a successor until then...