Lads, people need to read through Bruce's answers before asking him to clarify stuff, he's answering the same questions over and over again.
If you have Odyssey you have access to all of it. Odyssey and Horizons 4.0 (which are part of "live") and Horizons (3.8 Legacy).
If you do NOT have Odyssey, you have access to both versions of Horizons, 4.0 (live) and 3.8 (legacy) regardless if you currently own it, or buy it in the future.
Legacy and Live will be separate from each other as of U14. Legacy will no longer effect the "live" BGS nor will it receive narrative content. Nor will anyone have access to the servers for mods and messing. (At least not now)
Basically, there's no longer a reason to use legacy unless you want to be a holdout for whatever reason. (I'd have just binned it, myself)