I thought it was always clear that the 4-year lifespan was specific to Nexus 6 models? The N6 have shorter life spans, and fake memory cushions.
I don't think the standard Nexus 6 models had memories, just Rachel at that point.
The N6's were too childlike in their emotional responses, highly intelligent but only 2-3 years or so at the time of the film. If they'd had memories they wouldn't have struggled so hard.
The photos Leon tried to recover were of the Replicants together, right? Nothing to do with his own false memories?