Hi,
I have worked for US based company for 5.6 years. I have experience in Linux, Perl, Php, Mysql.
I have done this type of project in my job period.
I have some queries about your project:
1) Does all of 4 (A,B,C,D) database have same structure (schema)?
2) Does data just need to copy from 3 DB and copy in 4th DB, means all 3 DB have unique data?
3) Does need to remove duplicate records?
So review my proposal and if you like it than we can discuss further.
Thanks
Rohit