staining 2x6 old and new
I wonder what the texture of the original wood is. If it is rough sawn or smooth?
if the the original wood is rough, the 4 year old and new 2x6 is smooth.
I would take a 4" grinder with 80 grit paper on it and just lightly sand the surface.
Just touching it and run in long strokes will roughen it up and remove the darker color.
From the floor, would look more like rough sawn, little practice with the grinder and will get good results.
Otherwise just use a orbital sander on it with 100 grit paper and will lighten up the older wood, enough to match closer then it is now.
I would recommend using a good oil on it, just a natural color (prefer sikkens) Will look pretty white and blah when applied, although it does have some color to it.
Give it 6 months and will darken with age and look beautiful, after a year it will all blend in with the 4 year old and the recent new wood.
So you do not have to sand the wood perfectly white again, just get what comes off and closer then what it is now.
All wood will stain different anyways, some grain will be darker then others, even if the wood is all one species like pine. So the darker area will look natural to others, while you know it is the aged color, others will not.
If it was easy, we would not call it work!