Paul,
My 15 x4 and 15 x10 Cragars have a different center than the 14" centers as they are 1" taller. I would suspect the Keystones you have are 1" less tall than than the comparable size Keystone 15 inchers
1.If you do modify them to 14 x 4 there is no "skinny" tire out there that will look right or is even available that I am aware of.
2.If you do find a 15 x4 rim that will work on your 14" this is what you will probably end up..see picture below of late 1970's Cragars 15 x7 using what i believe is 14" centers. Notice the extra wide "kickup" of the steel rim so it is able to meet the 14" center. Looks unattractive IMO. I thought these were hideous back in the late 1970's when Cragar did this.
I'm not 100% sure this is what Cragar did but the earlier 15 x7's and later 15 x 7's did not have this extra wide portion on the rim. Cragar ran this rim from the late 70's to early 80's and then dropped it and went back to standard 15 x7 rim...
3. Early 15 x 6 , 7 etc" Keystones are on eBay every so often and the 15 x 4 steel rim was just a universal rim of the day with the same kickup and same center diameter and they are still available.
4. If you are going to go through the expense of getting the rims modified, hoops chromed might as well do it right and go with 15" plus if you ever want to sell them you will find a 14 x4 rim will have a limited market.
http://www.ebay.com/itm/150689634189...3D1&_rdc=1