два - двое
три - трое
четыре - четверо
пять - пятеро
шесть - шестеро
семь - семеро

That's about it I think. The reason why those "peculiar" numerals exist is that those numerals serve a little bit different purpose. You see, четыре, for example, is just a numeral. Четверо is not just a numeral, it's a collective numeral, these numerals are used to define a group of things as a single unit or entity if you will, whereas regular numerals describe how many of something there is. This "something" is key there, you don't usually see regular numerals used without another word after them, the word that they modify.

e.g. четыре человека. If you just say четыре that would just mean the digit 4. But you can just say "четверо" and that alone might easily mean a group of 4 people, not just the digit 4.

Here is where the things get dicy though. These collective numerals are exclusively (or at least almost exclusively) used to describe a group of living beings. So you can either say:

четыре зверя or четверо зверей.

and that would sound normal, but saying:

четверо столов would sound extremely awkward.


Also note this:

Нас в семье четверо - is an okay way to say "our family consists of 4 people"

Нас в семье четыре - is an awkward way to say that but:

Нас в семье четыре человека - is again an okay way to say that.

Finally, another thing worth noting:

When it comes to words that only exist in their plural form, you just have to use a collective numeral before them (if available of course)

See here:

ножницы - doesn't have a singular form:

Saying четыре ножниц is wrong, but saying четверо ножниц is okay. Yet again, saying одиннадцать ножниц is fine, because you just don't have a collective numeral for одиннадцать.

Hope that makes any sense, m8.