Delete and insert large number of records from MyISAM table

I have a MyISAM table that contains 30-40 million records. The data is for different users. I want to delete data for a user(Approx. 8 million records) and then again insert data for the same user(8-9 million approx.). This operation can be done by executing a batch from different servers(4 servers). Also, two users can simultaneously perform this operation from two different servers for their own user id. Is there any best and fastest way to do this job and handle the concurrency issue? Any help is highly appreciated.