You, and many others of us, are lucky to have any discrete commands at all!
They don't come with the original remote controls, and if someone finds them and has them added to the Logi database that's purely a bonus that comes with having a Harmony.
There's no point in having a Harmony and not using Activities.
You set up your devices, then allocate the devices to activities, in a process that's just as simple and straightforward as setting up the devices was!
When you do set up an Activity that needs HDMI4 you'll find, after setting HDMI3, that there's an option to include Additional Commands When Activity Starts, which is where you can enter the InputNext that you need. You don't need any actual input "buttons"; the Activities do all the input changing for you.