every four years

[Anonymous]

Are these all acceptable?

1-1. The Olympics are held (once) every four years.
1-2. The Olympics are held (once) every fourth year. [We learns this is correct, but it's incorrect, isn't it?]
1-3. The Olympics take place once in four years.
1-4. The Olympics take place once per four years.

2-1. The festival takes place (once) every two years.
2-2. The festival takes place (once) every second year. [We learns this is correct, but it's incorrect, isn't it?]
2-3. The festival is held (once) every other year.
2-4. The festival is held once in/per two years.

Thanks.

[Anonymous]

Are these all acceptable?

1-1. The Olympics are held (once) every four years. (correct)
1-2. The Olympics are held (once) every fourth year. [Yes this sentece is incorrect. The answer is 'once every four years]
1-3. The Olympics take place once EVERY four years. (sounds more natural)
1-4. The Olympics take place once EVERY four years. (natural)

2-1. The festival takes place (once) every two years. (correct
2-2. The festival takes place (once) every second year. [You're right it's incorrect. It should be 'once every two (2) years)
2-3. The festival is held (once) every other year. (I suppose this is correct but it sounds unnatural)
2-4. The festival is held once EVERY two years. (This sounds more natural)