4 is normal. It's somewhat complex, but there's a base amount between levels. There's then 3 things that can adjust (lengthen or decrease) that base amount:
1. How often you've gotten that question correct/wrong in the past. Example:
You started studying ここ, and you've gotten it up to level 3 without ever getting it wrong.
You started studying そこ, and you had a lot of trouble (missed it several times) before bumping it up to level 3.
In this case, the site presumes that you know ここ more than そこ (even though they're at the same level), and it'll take longer before it's quizzed again.
2. When the other vectors for that term are to be quizzed.
Let's say you're studying 住む and you've got kana->def, kanji->def, and def->kana. If you answer a question about kana->def correctly, it'll schedule when you study it next, right? However, it tries to make some allowance (to a certain upwards limit) to keep that question away from when the other two vectors will be studied. In other words, the site wants to minimize short-term memory interference. I'd prefer you to study this way
Q--Q--Q--------- (where Q are question days, dashes are no questions)
then this
---QQ-Q--------
3. There is a small randomizer that is used to help spread out future terms. As you get higher and higher, this helps lower those giant 'lumps' of study terms that cover only a couple days (and push them out across several more).