ci3 vs ci4 database library

Hi !   looking to implement ci database  library into an existing "oldie" project , still not decided whitch one to implement  !  
Main "advantage "  of ci3 is reusable query builder ? 
 same qb class for many db calls inside same model  ( ci3 version ) vs many qb class  per model &  db call ( ci4 version )
What would be the main drawback  on keeping  ci3 db ?   
What advantages   has ci4 db library ? 
 Benchmarks betweeen   db libraries ? 
Any suggestions ? 
Thanks !