Picking a Particular field from a Doctype

I want to pick a particular field from a doctype. e.g. I create a new field and link to Doctype by specifying the exact doctype in options.

My problem is there are 10 fields in the doctype I am linking. i want to pick only the 4th one or say the 5th one from the list of 10. How do I do that.

Hi try this ,