Some of this might be BS, and more 'how I see it', than anything they've ever done, officially, but I see it as each trying to keep a claw on someone each wants, if only to slap the others across the face. Slaanesh, ecstatic at this excess of blessings, throws her power in, to push that overabundance even higher, as well as she has her own interests, and then Khorne, knowing that anyone so ampped will be very powerful, and drawn to conflicts in the greater galaxy, throws in his two bits (I see him as more frugal in his blessings), thus we get a Daemon Prince of each, and it COULD still drift more toward any one, which would please that one even more. Upon seeing this, Nurgle doesn't want the things in this individual to change too much, and doesn't want to lose a prospect so interesting that he, AND Tzeentch, both had an interest in, so he throws in his blessings. For fun, I'll assume Tzeentch starts things off, as he seems the most likely to 'cause change'.
When the time comes to elevate them, each god contributes, not wanting to be left out. They all offer 'blessings', trying to woo the individual to their way of thinking, and ever-greater acts of depravity. Basically, each God wants that soul, and each is keeping an eye on it.
