Get to drop in (4,2)
I believe the answer is:
come by
I believe this is a double definition.
'get' is the first definition.
(coming by is a kind of getting)
'drop in' is the second definition.
('come by' can be a synonym of 'drop in')
'to' acts as a link.
(Other definitions for come by that I've seen before include "Manage to acquire" , "Encounter" , "Manage to obtain" , "Obtain accidentally - visit informally" , "Find" .)