I used to use these fellows. Quick service and good prices on 4x5.
Perfect Image Incorporated, 1550 S Mount Prospect Rd, Des Plaines, IL 60018
1550 S Mount Prospect Rd, Des Plaines, IL 60018
Local rumor is that they are back in business after a shutdown but I can't verify it without a call.
Gamma Imaging is the big player in Chicago: http://www.gammaimaging.com/ . At $2.80 they are not bad if you are in the area. If you are going to have to mail things out I have used River City Silver with good results.
BTW: All my 4x5 color has been using negative film, not transparencies, but the fellow who pointed me towards Perfect Image only did slides.
The multiplier? The problem here is that the formats do not have the same ratio of length to width. If you use the diagonals, 1.6 for the 6x4.5 and 3.6 for the 4x5 will get you in the ball park. If one dimension (length or width) is usually more important to you then use that ratio instead (remember that the medium format and sheet film image area is a little smaller than "advertized").