Frequently used in Belarus. You can find digital materials and electronic supplements through official educational portals.
A modern series for schools with an emphasis on the German language. Where to Find Solutions (Reshebniki)
The official publisher's site often has useful articles and teacher's manuals that act as keys for the exercises. Quick Tips for 4th Grade
The most common series in Russia. Solutions often cover the textbook parts 1 and 2 as well as the workbook (Arbeitsbuch).