I realise some people only play the one champ, and ragequit if they don't get to play it, but others like to switch it up. To you guys saying 1 page is the greatest wealth ever known and more than you could ever dream, consider this:
The prebuilt sets don't allow you to change anything. Since there are 5 players and 5 roles there are 5 rune trees to choose from, hence 5 prebuilt sets. If you want to have some fun with your build, fine tune it for your champ or playstyle, to counter enemies or just try something out, you have 4 pages. There are 20 permutations of primary/secondary runes. Only 4 pages. One more and you could at least have an alternate setup for each type primary rune set. But no. 4? Weird.
There were 30ish pages available for masteries. I guess 4 is technically a compromise between the previous amount of starting rune pages (2 or 3) and mastery pages (a lot more), but so, so technically. Thankfully what the new runes actually do is slowly settling in, so it's getting easier to successfully change them before a game.