Does it work on an oldish Samsung tablet TAB4 (about three year old)?

Well, it works on Android 6 (Marshmallow) and above.  I just looked up the TAB4 (http://www.samsung.com/uk/tablets/galaxy-tab-4-7-0-t230/SM-T230NYKABTU).  If you buy one new, it will come with Android 7.  So, maybe, your tablet is either running 7 now or could be upgraded to 7?  If so, it should work on that.