The database structure in 4.x is different, thus Surveys are not downward compatible. You can import LSS files created in 3.x into 4.x. But you can't import files created in 4.x into 3.x.
Help us to help you!
Provide your LS version and where it is installed (own server, uni/employer, SaaS hosting, etc.).
Always provide a LSS file (not LSQ or LSG).
Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.
The following user(s) said Thank You: DenisChenu, lewoptics