I get most of my parts from a Amish down the road from me. He makes the outside grass board and pitmans in his shop and is able to order in most of the wear parts. (Sickles, guards, knives, ect.) Granted, they don't have the pretty green paint but they work just as good. He is also very fair in his prices. If you like, you can send me a e-mail of what you are looking for and I can pass it on to him or you can contact him at:
Mast Buggy Shop
W5253 Barry Rd.
Dalton, Wisconsin 53926
I'm in the process of scanning my #4 manual to post on a web site I'm building. What area of the mower are you needing parts for? I'll do my best to e-mail you the pages you need.
As far as I know, the #4 was the last of the horse drawn mowers made by John Deere before they went to the #5 caster wheel tractor mount mower. Sorry but I don't know what years Deere made the #4 and Big 4 mower.
Hope the above information helps you out.