LR4 will remain as a separate app and your photos will be safe. When you install LR CC you will get the option to convert the LR4 catalog which will preserve all your library links. You can choose to convert during installation or you can do it manually afterwards.
So keep LR4 installed, at least for a period until you are completely happy with LR CC. You can then uninstall as any work you do going forward in LR CC won’t be backward compatible.
Thanks, will try this in the next few weeks once I have OS X updated
Actually you only need to upgrade that older Mac to 10.9.5 for both LR and PS CC to work on it. You do NOT have to update it all the way to El Capitan and I suggest you don't even try.
But the first thing you should do before upgrading OS X is make a complete system back up using one of the Cloning software packages available, Carbon Copy Cloner or Super Duper, along with doing a new Full Time Machine backup.
That way if for some reason you don't care for the newer version of OS X you can safely revert that older Mac back to 10.6.8 and keep all your apps and settings in tact.
Take note that any of the newer version of OS X, Lion on upwards, do not support Power PC apps. Snow Leopard 10.6.8 was the last version of OS X that supported Power PC apps. So if you are running older software that was designed for the Power PC CPU they will no longer work once you upgrade OS X.