Mac OS X can't be installed on this computer when I tried to downgrade from 10.9.4 to 10.9.3
Apple's OS doesn't downgrade. You will need to back up your files and apps to an external location and then wipe the main partition using recovery mode.

At that point, you can run the 10.9.3 installer - but again, the migration of the user data will be by hand since the assistant won't load the newer files onto the older OS.

Your best bet might be to install 10.9.3 on to an external drive and test changing the home folder location from the new install to point at the old drive (after it's backed up of course) and making a list of apps that have failed.

At that point, you can ask one by one for help if a quick search doesn't yield an answer to those specific issues. Most apps should work fine with such a small regression.