Replaciing my Gen. 3 SSD with a Gen. 4. in a 2014 MacBook Air?

I would recommend reading up a bit on the PCIe/NVMe specs as well as review this great guide! The Ultimate Guide to Apple’s Proprietary SSDs

Your 2014 13" MacBook Air has a PCIe 2.0 x2 SSD's (two lanes) the newer MacBook Air's SSD's are PCIe 3.0 x4 (four lanes)

BeetsTech guide states the 2014 13" MacBook Air has a PCIe 2.0 x4 interface even still you would assume the PCIe 3.0 x4 drive should work with the PCIe 2.0 x4 interface on the logic board it appears it won't use the four lanes so you don't really gain any real performance.