My predictions for how it goes if the SH's were to face the four yonko;
1. Big mom is next, but no real fight. Luffy will get in some tussle, but will ultimately either beat big mom in an eating contest, or impress her so much with his ability to eat she respects him.
2. KAIDO, this will be a fight... I haven't the foggiest to hownit will go or what will occur.
3. BLack beard, there will be no meeting shanks, or fight/alliance with the red hairs, black bears will have killed shanks by this point, and will fight luffy at rafel