OK Pause for breath dude, try using punctuation too
You can get homebrew software that enable you to watch youtube videos on your psp.
It is not possible to watch youtube on official firmware PSPs, so you need custom firmware to enable the use of homebrew
OK?
You need to downgrade from version 4 yes, its called downgrading because you will undoubtedly be on a lower custom firmware version than the most current Official firmware, but thats a minor detail... The only way you can switch to custom firmware is to get hold of a pandora battery, get hold of the neccessary files and this will enable the update to commence. You will then be on custom firmware.
At this point the world of homebrew is open to you, you can download psptube, connect to you wireless internet connection and watch youtube vids
which
I think is what you were saying - but who knows