Thanks - so it is very limited, like I thought. Only 4 bus routes.
That's not "limited"! There's only four parks, and there's ample transportation between all of them! Not to mention, there
are other routes besides the recommended ones in the bit I clipped. Disney was just giving recommendations.
Where do you want to go?
Epcot to the Magic Kingdom: Bus/Monorail + Monorail/Boat/Bus
Epcot to Animal Kingdom: Bus
Epcot to Hollywood Studios: Boat OR Bus
Magic Kingdom to Epcot: Monorail/Boat/Bus + Bus/Monorail
Magic Kingdom to Animal Kingdom: Bus
Magic Kingdom to Hollywood Studios: Bus
Animal Kingdom to Magic Kingdom: Bus
Animal Kingdom to Hollywood Studios: Bus
Animal Kingdom to Epcot: Bus
Hollywood Studios to Magic Kingdom: Bus
Hollywood Studios to Epcot: Boat OR Bus
Hollywood Studios to Animal Kingdom: Bus
Proof (from EasyWDW):
How many more bus routes do you want???

The only hopping that's even remotely complicated is the Epcot/Magic Kingdom route, because it requires a transfer. Otherwise, you can take a bus anywhere you like. Even between Epcot and Hollywood Studios, if you happen to be a hobbit (and fearful of boats).