This may not be applicable to ALL problems with tilemaps in 1.4, but I solved the issues I was having by building a new project in 1.4, creating a new tilemap node in a new scene, and importing my scripts/ graphics etc.
After I upgraded to 1.4, I opened my project and had problems with the tilemaps. My problems were z-order related (sprites not showing on top of map), collision, and lines between tiles. I eventually opened an "Examples" project that I had created with a previous version (not sure if it was 1.2 or 1.3) and opened the tilemap example. It had some of the same problems.
I next created an examples project with 1.4, and the problems were gone. So I came to the conclusion that something changed with tilemaps in the 1.4 engine, and corresponding changes were made in the 1.4 editor so that the assets automatically created would be compatible with the new engine.
I haven't taken the time to go look at the changes on Github, but it seems that tilemap nodes created with 1.4 don't have the problems.