Two British men, aged 65 and 49, a British citizen of Kuwaiti origin, 42, and an Egyptian, 33, were arrested.
It follows raids in Birmingham, Blackburn, and Wiltshire on Tuesday, the National Crime Agency said.
All four were arrested on suspicion of conspiring to facilitate illegal immigration and have been bailed pending further enquiries.
The operation, which was carried out in co-operation with the French authorities, follows a report of suspicious activity involving a rigid hulled inflatable boat in Ramsgate, Kent, in September last year, the NCA said.
The agency said it was suspected that the boat been, had or was about to be, used for the transportation of migrants illegally from either France or Belgium.
Money, mobile phones and computers were also seized by investigators.
Peter Smith, branch operations manager for the NCA, said: "The arrests are the latest stage of a complex investigation into the activities of an alleged criminal network involved in moving migrants across the English Channel using small boats.
"Our inquiries will continue and we are now analysing evidence we have recovered as part of this phase of work."
Four men have been arrested as part of an investigation into alleged smuggling of migrants across the English Channel.