Plumbing UK sink waste
You want something like this.



This is a rubber connector which goes in the top of the 110mm pipe, and the 40mm pipe slides in to it.

Yes, the connection needs to be sealed to seal off smells.  If the 110mm pipe gets blocked (unlikely, because any blockages are much more likely to form in the 40mm pipe), you can easily disassemble it.

Note: The link is to Screwfix because that was the first result I found.  I wouldn't actually buy from Screwfix.