Cathi Szulinski did a lot of research and wrote up her findings:
This extraordinary story was brought to my attention by an intriguing footnote. It told me only that four young Russian men had been sent to England by Tsar Boris to learn English, but that the Time of Troubles had prevented their return.
What initially intrigued me about this remarkable early foreign-exchange project were the individual stories. If the young men had not returned to Russia, then what had become of them? I decided to try to find out. The quest has led to some surprising places. …
It’s long, but if you have any interest in 17th-century England, well worth your attention. (I found it in an annotation at Pepys’ Diary.)