I can’t speak for adara/ your plum. Since im still searching for adara (it’s not as widely available in the EU. (i don’t have experiance with it since i don’t have it 
)
But if had apple scions that i kept for 6+ months in the fridge as a experiment. (back of the fridge set at 2c, so they where between 0 and 2c i guess) (2c = 36f)
All the white stuff is callous. But since both scions are parallel, there is no hormonal gradient (more cytokines on top of the bottom part of graft. and more auxines at the bottom of the top part of the graft) and thus little to no differentiation happend. And the callous stayed callous.
This is what happend.
Kind of amazing how much new callous tissue a scion can make just from stored moisture and resources.
If i where to place an educated guess. I would assume the callousing proces would slow down considerably. But would likely not be damaged. (as long as it does not dry out or got infected by fungus) if you put your freshly calloused grafts in the fridge.
callousing happens way faster at higher temperatures. And enough moisture. But not to much moisture.
My grape rooting method apparently was ideal for callous formation. As evident by the callous formation splitting this grape cutting’s bottom like a banana.
I have not searched for formal research. But if seen anecdotal lists from experienced people on this forum. And those seem trustworthy enough to me.
I personally would either do summer bud grafting on adara shoots. And than treat those shoots as normal scions. (cut when dormant store in fridge, graft when temps outdoor are high enough/ when rootstock starts to wake up)
Or i would do all the grafts on the same day. (graft the bud on adara, then W&T adara on stock.) Or double W&T.
The whole thing with grafting is, your severing the connection between the shoot (scion) and roots. And since the shoot evaporates water, that it normally gets from the roots, it’s vulnerable until a new vascular union has formed, and it has access to water from the roots again.
To help us with this. We ideally want the rootstock “awake” and actively growing and forming the new union with the scion, before the buds on the scion start to leaf out and increase the evaporation.
That’s why we store or scions cool. We want or scions to think winter is “ending” later, than the rootstock. So the rootstock wakes up before the scion. And the rootstock does most of the “heavy lifting”
I’m not sure how your scions are, dormant wise. i suspect 4 days at room temp could be fine. But id definitely adjust if you see the plum buds brake.
Another thing you could try. is locally heating the graft union, but not the bud. I think barkslip has a tutorial about that. Maybe he can give you better advise on how much heat for how long, will not wake up the scion.
For me though. I’d just store your plum and adara in the fridge and do all the grafting on the same day when your rootstocks outside are waking up.
i hope that helps. And was not to confusing.
