Usually we test in-dojo three times a year. These can go as far as shodan I think. During Easter we have a seminar with a japanese instructor, were you can go for dan-grades, and in the autumn we have a national seminar where you can go for 3. kyu and above.
We usually tend to go for at least a year between 1. kyu and shodan - but not allways. It's the formal requirement, but in the end it depends on so many issues.
I see no problem in taking 5. and 4. kyu in one year.
Just don't focus too much on the gradings. It's the time between them that should count the most