It all depends on what you are doing.
If you are exporting photos from LR 5 as TIF or JPG and importing into LR 4, this works, your edited photos will appear in Lightroom 4
If you are moving the catalog from LR 5 to LR 4, this will not work.
If you are writing the edits to XMP, and re-importing the photos into LR 4, be aware that some new features in LR5 will not be understood by Lightroom 4.
Best for everyone to use the same version of Lightroom if possible.
You can certainly do it in such a way that there are no problems, *iff* you know what you're doing. Since you're asking this question in the first place, my advice would be:
Download Lr4 and use it to edit your Lr4 client's photos - that way, you are guaranteed no problems.
To be more specific - Lr4 shouldn't have any problems *importing* Lr5-edited photos (with Lr-saved xmp), but whether they will look the same depends on whether you used Lr5-only adjustments, and to be frank, a bit of good luck (Adobe did not design it to work that way (editing the same photo in different Lr versions), but is pretty good about ignoring things in xmp that it does not understand..).
Do you feel lucky ,