It was very odd. I'd built a couple who'd had 6 pairs of twins and I really didn't want to lose the save. I went to Manage Worlds and decided to give a few of the pre-generated families in Sulani some pets to breed with mine hopefully. There I was able to save without issue. After that I went back to my family and voila, saving was working just fine again. I was hoping someone knew what the code number meant was wrong.
I doubt it would have been a problem with my system storage, it can hold a full terabyte and I've only had it 6 months. Though I do wish there was a way to delete old Sim saves that I'm no longer playing, some of them I had to move away from because they lagged my console something fierce. The bigger my house the longer it takes to load. >.<
Thanks for mentioning the PS4 reset though, I hadn't thought of that kind of reset (the 7 second hold) and I'll try that next time I'm at a roadblock. :D
~Tyger <3