Sqlite way to copy in a database only the distinct data present in another database

Say I have two databases with the same structure but containing different data A containing 1+2+3 as data spread on different tables; B containing 1+2+3+4 as data spread on different tables;

Is there a way to find 4 and copy it into A?

Something like copying the distinct data only.

I was thinking about a way to 1) compare A and B; 2) find the differences; 3) insert the differences in A;

dump A - dump B - diff the dumps - patch diff to A - feed to sqlite

but this approach doesn't seem to work. patch gives troubles (offset lines)

Any idea? I have very little experience in programming, any suggestion is appreciated. Thanks