Hello,
when sending off print jobs, you can't be sure which result to get - except for expensive CMYK contract proofs. The reason is that for RGB pictures always a "perceptive" color conversion to the photo printer's color space is done. Converting to CMYK is not a perfect choice either as it limits color gamut and maximum intensity of black.
I'd do the following:
1. adjust your monitor using a gamma test picture
2. create a profile for your desktop printer using our profiling service
3. print a few sample pictures on your desktop printer
4. send the same pictures to your print service and compare the results, so you know what to expect in the future
I suggest to send JPG or TIFF in sRGB or AdobeRGB color space. Make sure that you select the correct color space in TurboPrint when printing the pictures on your iP4000.