Before you install 10.10.4
Wait. Once it is available at http://support.apple.com/downloads it is more likely to be better. Though you still want to wait.
Backup your data!
Backup your data!
Check with software and driver vendors that all your applications are known to be compatible with it.
Backup your data.
Then upgrade. Don't upgrade before. Don't add system utilities without asking here first if they are needed, and never before backing up your data.
OS X Yosemite (10.10), * I may get paid from macmaps links