If the motherboard of your computer takes DDR3 then that’s what you’re stuck with, if it’s DDR4 then you need those instead. They’re not interchangeable without swapping out the entire motherboard.
Main changes are …
Speed – DDR3 topped out around 1866 MT/s, DDR4 started at 1866 and is now up to 2667. There are several more speed increases planned on the industry roadmap.
Lower voltage – equals lower power. DDR4 was initially introduced in the server market to reduce data center power usage.
Higher capacity – DDR3 mainstream topped out at 16GB (workstation/server space). Maybe some 32GB but not much. DDR4 started at 8GB and currently supports up to 128GB DIMMs.
No future development for DDR3 – means than manufacturers are exiting the business and converting their capacity to DDR4. DDR3 costs are going up and future cost reductions will only apply to DDR4.