how do i connect a 4mm motor shaft to a 3/16 in flex shaft

u sure its not a .150 flex with 3/16 drive shaft on the end?

cuz if so this would work [link=http://www.offshoreelectrics.com/proddetail.php?prod=oct-ocfhe4mm15]coupler[/link]

Jeff Wohlt at [link=http://www.rcraceboat.com/]rcraceboat.com[/link] can make it for you. I've had him make me many unstandard sizes in the past and I've been happy with the results. Good price too.