Can 3D building shapes be previewed?

There are different options for you to preview 3D renderings:

The Kendzi 3D plugin for JOSM is probably the best choice for your purpose. It allows you to instantly view your editor contents in 3D and can easily be installed from within JOSM.

Alternatively, OSM2World (Wiki) can also be used offline. It's a stand-alone program and a little bit harder to set up:

• Download and unzip "latest build" and "example texture selection" from the download page
• Call the appropriate script for your operating system (.bat or .sh) in the OSM2World directory from the command line, and add the parameters --config texture_config.properties --gui, with "texture_config.properties" pointing to the file of that name from the "example texture selection" archive you have downloaded previously
• From the program window, you can now open the .osm file you want to view (it also accepts saved files from JOSM) and reload it after edits

It should be noted that both programs support some features not (yet?) present in F4 map - after all, they have been around for a few years already -, and interpret some things differently. For you, that means that there is no guarantee that F4 will render buildings in exactly the same way, but I still think it is very helpful to have the instant verification of your 3D mapping efforts.