Permutations

I can't remember when I learnt permutations. Given 4 constants - a, b, c, d - how many combinations will there be? 4 x 3 x 2. But I don't want similar pairs, eg. aa or bb so that means 4 x 3. And I don't want flipped pairs that are similar, eg. ab and ba so that means 2 x 3. So, how did I end up with the four below...LOL! Can you find out which are the 2 other combinations that I didn't make?

I'll get round to it soon. Not now....on to other things :-)
Have a blessed day!