With DDR3 I used to buy RAM "kits" one at a time to build my system up over time as and when I could afford it. I would make sure to buy the exact same "kit" from the same provider to avoid issues. E.g. buying 4 lots of 1 x 8 GB sticks or 2 lots of (2 x 8GB) etc.
I was told that I cannot do this with DDR4 and that one has to buy all 4 sticks in a single kit at once. Is this true?