My wife had a tab3 that she loved until it developed a weird fault where it would turn itself off and resist all efforts to turn it back on again. After much shouting John Lewis finally accepted it was defective and offered the refund they promised.
When we moved US-wards she wanted another tablet so we got her the tab4, which she loves.
From what I can see the differences are minimal. I haven't bothered to check the technical specs in detail but I'd expect it to be little more than a slightly faster processor, maybe a little more memory, slightly better camera and so on. If I had a tab3 I don't know I'd bother upgrading to the tab4, but if I didn't have a tablet at all the tab4 looks like a reasonable one to go for.
Check that the resolution is good enough for what you want to do. I like to work in microdot so the 10" tab wasn't really up to the job for my purposes, so I paid the extra and got what I think was called a Note Pro which has a much higher resolution screen.
