Well, put simply, a “4F” is an 0-6-0 goods steam locomotive which has its origins in the range of smaller 0-6-0 designs initiated by the Midland Railway company in the late 19th Century.
“4F” is the power classification assigned to the loco. “4” indicates a loco of medium power (in the range 0 – 9), and “F” stands for “Freight”. (Even though the railways commonly refer to “freight” as “goods”)
http://en.wikipedia.org/wiki/LMS_Fowler_Class_4F
If you want to know more about 4Fs, why not take a look at these excellent and informative books.
A defence of the Midland/LMS Class 4 0-6-0 by Adrian Tester
LMS Locomotive Profile 10 – Std Class 4 Goods – By Wild Swan
LMS Profile 10 Pictorial Supplement 0-6-0 – By Wild Swan
Fowler Locomotives – A pictorial history by Brian Haresnape
LMS Locomotive Design and Construction – A.F. Cook
Raising Steam on the LMS. The evolution of LMS Locomotive Boilers – A.F. Cook