I'm afraid, looking at 4x6 print is not a good way to judge sharpness. Just about anything can produce sharp looking image on that size. In fact, one of the tricks for using slightly out of focus image IS to print smaller.... For 35mm, you would probably need to enlarge to 8x10 to better see focus problems at corners. Or, you can get a lope and see negatives under magnification.

I use a 10x to 40x stereo microscope for this purpose (yes, an overkill! but I already have it) but you could use 8x lope as well.