4) Ensure that my players get less XP, i.e. only 1/5 less XP than normal. If I achieve this over a 4 PC Group or a Group of 5 Players that get less XP or lesser combats should be the same.
I'm completely OK with 1) to 3) but what about 4)? Does this really work? What about my actual group which is at the end of Thunderspire Labyrinth and is level6. I cannot steal their levels. But this should stable over the next levels if I give them lesser XP.
Personally I halve monster hit points, award full monster XP, and typically run adventures for my group within a level or two of recommended level. I don't really do (4) - if anything I give bonus XP. My group are not super-optimised powergamers, though. If yours are, you may find that giving full XP for half-hp monsters makes things too easy, in which case you can try 2/3 XP. But cutting out boring fights while giving full XP for the battles they do fight is a better approach, I think.