There are a couple of ways to do what you want, 1.) is using Adobe Illustrator, the other is using 2.) Adobe Photoshop:
- how can I include a border on every Din A4-page to make sure that everything lies within the printable area
Open the Inkscape file using 1.) Illustrator > View > Show Page Tiling; use the Page tool and print the selected page area; in other words, align the Page to where you want to print, but instead of printing > Save As > PDF. 2.) Using Photoshop, open the Inkscape file as a .tiff @ 150ppi @ RGB ( or CMYK ) > Set the Rectangle tool to a size slightly smaller than A4 ( you will have to determine how much bleed you will want to include in the capture > in PS use guides to divide the original poster into A4 sections > use the Rectangle tool to select each tile > Copy each tile and paste into a new PS document > Save As > Photoshop PDF.
- without shrinking the original size of the document
The methods above will not shrink the original.
- including marks ON the sheeds, ideally numbered, showing where to cut and stick
You would be better off using Illustrator for that > Open PDF created above ( one of tiles ) in Illustrator > you should see the page margins used in the print-to-PDF op > manually add trim marks and tile number.
- as it is not a poster but a pattern it would be fine seeing these marks even after having cut and sticked
Yes, but you probably won't because each page will be trimmed to assemble the final poster.
Thank you very much for your help, this is a great support!