Every little thing you want to do takes tons of background information. How do I load a particular agent in a particular slot? Do I have to select a kit/program that USES that particular agent then change everything individually to get what I want? Can I not just load sounds onto pads and make my own kit? The manual is actually less than useless as most of the time it starts by explaining something completely different and then finally gets to the thing you are actually try ing to achieve and simply says " Do this…"… If I could “Do that…” I wouldn’t have just spent several hours trawling through the manual trying to find out how to “Do It”. 
Hey Deety,
Indeed, it is sometimes necessary to read, search on the internet or to consult this forum in order to obtain the maximum information to be able to run all this.
For GA4, it is possible to have 4 Kits at a time. You can select a kit and load a program of your choice. Each pad of each kit can be changed individually. The EDIT MIXER and PERFORM pages allow you to make all desired changes and in general, all this is quite user friendly and intuitive.
It is also possible to create your own drums kit, simply take a sample that you have created yourself or from a database and drag this sample on one of the 128 pads available and make the necessary adjustments In the pages mentioned above so that all this corresponds to your expectations. It’s a lot of work, but it’s exciting. It is impossible to achieve something of quality without putting the necessary time and it is sometimes many hours.
Don’t let go!
I hope that has been helpful.
Renel