Mongodb Migration from large number of database

Hi Team,
I just wanted to migrate my whole number of databases to single db database .
Like i have 4 application db : A,B,C,D…Now I just wanted to move application A db to different mongodb instance without downtime.

So what is the right way of migration:
1.I take backup of A and dump it on another instance of mongodb.
2.I create another instance as slave of primary having 4 databases(A,B,C,D):(Question :right???)

3.Now I will promote another instance of mongodb as primary after checking rs.printreplicationinfo()