First let me say, I don't work much with slides, but I would probably approach it this way.
You are going from a 2x2 slide to a 4x6 print and there is no proportional sizing or aspect ratio that will work easily. So I would first try to set the scanner and let it produce a 4x4 scan at 600 DPI (or bump that up to 1200 DPI if the initial results are not good). The idea is to get something closer to the needed 4 x 6 size. Then I would use the CROP TOOL in
PS to the 4x6 or 6x4 size and crop at 300 DPI.
Do any cleanup, touchup or corrections and save your work at this point as either TIFF or PSD. Archive to CD or backup server/hard drive.
Now you can save a copy of your work as high resolution JPEG for sending out to print at Shutterfly. Or, put them on a thumb drive and take them to CVS or local film processing outlet.
Hope this helps.