A good question because 4R Yellow Label has a distinctive taste, one that doesn't really mix well, whether as opposite or complement, with the bourbons of other companies.
As I result of this, I generally will blend it intramurally so to speak. I just added some 4R SB to a bottle of 4R Yellow Label and it made it stronger and richer but preserved the signature floral character. (Any 4R SB will do I find, it doesn't really matter which, they are not that different). Mariage and its brethren will work too but you get more oomph from the SB.
Here is another way: blend it with Bulleit. Bulleit comes from some of the same bourbons in 4R SB. It's all of a piece. So 1:2 4R Yellow Label to Bulleit and you get a still potent whiskey with a lot of floral and yellow fruit character.
I don't want to rule out non 4R bourbons for the blend, I'd have to think more about it, but the above is generally what I do and the results are excellent - for drinking.
And as a follow-up I too have not found anything that mingles with 4R yellow other than other 4R or Bulleit. I've tried ODG BIB, HH BIB and Old Charter 10 but meh on all. Sweet vermouth and bitters is the other way to go with the yellow imho. I remember tasting the yellow for the first time at the distillery, with Gary, and really enjoying it, but the subsequent bottle I bought was a let down and called for experimentation.