I've been looking around the internet, but I didn't find a clear answer stating the difference between 口 and 囗.
口: Kanji mouth
囗: radical box/enclosure

- If something is inside the square, is it always the radical box or can it also be the Kanji mouth?
- Nowadays, do Japanese even distinguish between the 2 squares?
- In RtK 四 （four) is described as: "mouth 口 and human legs 儿"
However the following website states that 四 actually uses the radical:
http://classic.jisho.org/kanji/details/四
Also Wikipedia has 四 as the radical
http://en.wikipedia.org/wiki/Radical_31?oldid=490587026

Could anyone enlighten me about this?

I don't know about Japanese people, but Heisig has a habit of assigning different keywords to variants of the same radical/primitive (for example 示 show becomes altar as a left-side like in 神). And when you put 4 and country side by side:　四国 there is a size difference, so it would logical in Heisig's approach to assign mouth to 4 and box to the other. Keep in mind his aim is not to teach you the traditionnal radicals, but to help you remember the shape of kanji.

The radicals in question are

囗　くにがまえ, which means country-enclosure because it is the enclosing radical of 国 (as well as a lot of other kanji of course, but 国 is the one it is named for). It is not a kanji in its own right. A かまえ is a container, so it always contains something.

口　くち、mouth which is a kanji in its own right as well as forming part of many other kanji.

What may be confusing here is that as a component くち is sometimes regarded as a box. 品 "goods", I believe, did originally represent stacked boxes for example

Even though it can represent a "box", as far as I know there can't be anything inside the くち radical. It is possible though for something to pass right through it, as in 束.

So the container in 四 is the くにがまえ element, though it might be called a mouth for mnemonic purposes by Heisig-sensei. Many of his mnemonics are not (and not intended to be) etymologically correct, even though quite a few are.

I am not a kanji expert so someone else may correct me.

If you want to understand the origins of the kanji, there are books like 常用字解 by 白川静 that you can read at some point. Until then, I don't think worrying about the difference is at all useful. The derivation of the kanji can be very obscure.　I'll try to remember to look up a few examples of this enclosure, but I think you should use whatever makes it easiest for you to learn the characters.

口 (mouth) is always drawn smaller than the くにがまえ.

白川 describes 国 as 会意, and lists 或 as the base character for 国, and then states that the the かまえ was added to avoid confusion when the connotation of 或いは was added (常用字解, page 209). He claims 或 represents a city (the square) protected by arms (the spear). The combination represented the armed capital of a country. The character later took on its current meaning of "country". So the かまえ represents a city. That's his opinion, anyway. Although that's interesting, I am not convinced that it helps most people very much with learning the characters.

Oops, I meant to add that 口 is described as a pictograph of a ceremonial vessel that was used for divination.

囗　くにがまえ, is never used on its own. There is always something IN it. That's why it's bigger then 口 (mouth), because you have to be able to put something in it.

So, the difference is nothing to worry about.

Also: lots of descriptions of kanji are only mnemonics, not an actual description of how they're put together.

四 (four) has nothing to do with 口 (mouth) and 儿 (human legs). It's only a way to remember it.
As far as I know, 四 is derived from a hand holding up 4 fingers.
Sometimes the real ethymology makes it easier to remember than all that fancy stuff to "explain" it.

Wow, I feel like an 阿呆 for not realizing the similarity between 國 and 域.

白川 lists 四 as a 仮借, or a borrowed phonetic character (常用字解, page 240). The 甲骨 version was four horizontal lines showing divination blocks, which makes sense. By the time of 篆書 style characters, it looked much like its current form. So he seems to list the current form as the 音符 from the character 口四, by which I mean a single character that I couldn't find easily in kotoeri. This character (or part thereof) likely represented something like a mouth laughing.

Looking these up is fun for me. Maybe I'm more than a bit warped.

As long as you can come up with that kind of remarks, I don't want you to be anything else but warped.