Should I upgrade from DDR3 to DDR4 or stick with DDR3?

Stick with DDR3 unless you’re building a whole new PC. If your motherboard supports DDR3, just buy another 8GB stick for $20–25—it’s cheap, works, and 16GB is useful for multitasking. The performance gap between DDR3 and DDR4 on older platforms is small, and switching means a full rebuild with a new motherboard and CPU. Future proofing with DDR4 on a dead socket doesn’t make sense.