yes, you can go directly fro 4.0 to 4.3. Just be sure the database
tables you use have the structure like in 4.3 and their version
number is updated as well in the database table version.
Then check to see if some parameters/functions need to be updated in
kamailio.cfg, then you are all set.
Many times it happens that I have to jump over many version with
upgrade and it just works by installing the new kamailio version
binaries and restart (e.g., edge proxies which don't rely much on
database interaction).