You can't import or combine projects in Premier Elements.
You can output a finished video from each of several projects and then join them in a new project.
Some find the H.264 MP4 outputs of sufficient quality that there is no visually noticeable loss from rendering. Others find it better to use a "lossless" codec to output from the sub-projects for the mater project. The lossless Lagarith codec is available as shareware on the internet. It's downside is that the intermediate files you produce will be large.
Thanks for your answer. I hoped there is a possibility to combine projects.
So I start to export my sequences with 4K-MP4 in high quality and these files are also not small.
I have no idea what "lossless Lagarith codec" you mentioned I could use for my 4K projekt instead.
Is the quality better than 4K-MP4 high quality?
Where can I find the codec?
Is Premiere Elements 14 able to read this codec during import to the final project?
Is there a describtion how to implement the codec in Premiere Elements 14?
I suggest you try the easiest way first. Yes, you output your "sequences". That word has specific and technical meaning in professional video production software and does not really apply here. In Premier Elements, you will instead export finished videos to be combined in a master project.
To summarize, output a finished video from one of your sub-projects. Pick output settings that match the source footage as close as possible. Open a new "final" project and import the result from your sub-project. Examine it for visual quality. Before you process all of your sub-projects, output a "test" sample of the final project and again examine it for visual quality.
The theory is that because you are double processing or "rendering" your footage twice, there may be some loss of quality. This is due to the efficiencies designed into the coding-decoding "codec" software. From what I've read, it used to be a lot worse than it is now. Current "codecs" that come with Premier Elements may not produce enough encoding or processing loss for you to see it on the screen.
"Lagarith" is not an Adobe product. It may not be commercial product at all. Reading suggests it was developed for making intermediate files for video production that will eliminate any encoding compression losses. You get it by finding it with a Google search and downloading it at your own risk. For fun, self education and personal entertainment, I tried it once to do what you are doing. My source footage was pre-4K, so not as good. Even with lesser quality source footage, it didn't produce results that I saw with my eyes to be worth the trouble.