In the military draft system, what did the classification of 4F stand for?

Dear clown123,

The Selective Service System classification "4-F" is officially
defined as follows:

"§1630.44  Class 4-F: Registrant not acceptable for military service.

In Class 4-F shall be placed any registrant who is found by the
Secretary of Defense, under applicable physical, mental or
administrative standards, to be not acceptable for service in the
Armed Forces; except that no such registrant whose further examination
or re-examination is determined by the Secretary of Defense to be
justified shall be placed in Class 4-F until such further examination
has been accomplished and such registrant continues to be found not
acceptable for military service."

The short version: A person classified F-4 is not acceptable for
military service in the United States forces.

Source:

Electronic Code of Federal Regulations: Title 32 - National Defense /
Chapter XVI - Selective Service System / Part 1630 - Classification
Rules
http://www.access.gpo.gov/nara/cfr/cfrhtml_00/Title_32/32cfr1630_main_00.html

Electronic Code of Federal Regulations: Title 32 - National Defense /
Chapter XVI - Selective Service System / Part 1630 - Classification
Rules / §1630.44
http://ecfrback.access.gpo.gov/otcgi/cfr/otfilter.cgi?DB=1&ACTION=View&QUERY=1630.44&RGN=BSEC&OP=and&QUERY=32&RGN=BTI&QUERY=709&RGN=BSECCT&SUBSET=SUBSET&FROM=1&ITEM=1

Search terms used:
draft "classification 4f"
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=draft+%22classification+4f%22&btnG=Google+Suche&meta=
4f "military service"
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=4f+%22military+service%22&btnG=Google+Suche&meta=
4f unable physically military
://www.google.de/search?q=4f+unable+physically+military&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&start=10&sa=N
classifications 4f "military service"
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=classifications+4f+%22military+service%22&meta=
"Selective Service" "4 F" military
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=%22Selective+Service%22+%224+F%22+military&btnG=Google+Suche&meta=
"4-F" "Not Qualified for Military Service"
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=%224-F%22+%22Not+Qualified+for+Military+Service%22&btnG=Google+Suche&meta=

Hope this answers your question!
Best regards,
Scriptor

Had I been the questioner, I would have given five stars to this
excellent and wonderfully informative answer. I cannot imagine what
the questioner found lacking.

Affirmative on that one