You are 4x as likely to lose 1/4 of the wedding photos if you use 4 SD cards. But you're 10000000x more likely to not get paid if you lose 100% of the wedding photos by using only 1 SD card.
This was my thought as well... I'd rather have a bigger chance to lose a LITTLE than a smaller chance to lose absolutely everything.
But then again you could use backups of each SD card as well and create a bigger buffer (same with wallets).