It's only my opinion but there isn't enough change on the SD4 for the 300 $ more price.
The SD3 can be upgraded for less than 300 $.
For example, buy a DS3 and also a E3D head, print the MK5 Lawsy and you will have something better than the SD 4. You can also change some original parts with cutom printed parts (the ones with linear bearings for example) it will cost only a few dollars and will give better precision for the head.
I think you'll better keep your money to upgrade a SD3 than buying a SD 4.