I’ve been researching the crap out of wraps so I’ll add my 2 cents to this old thread.
There are 5 main types of wraps (not 4) and a wrap is any Omaha straight draw that has more than 8 outs (which is the most you can have in NL with only 2 hole cards). There are 9, 13, 16, 17, and 20 out wraps.
16 & 20 out wraps use all 4 hole cards and 2 or 3 board cards
9, 13, & 17 out wraps use 3 hole cards and 2 board cards
To narrow down between these groupings
memorize the different wrap formats in this table.
Also, if you near the end of the board (some ace high or ace low straight draws), or if you have multiples of the same out card in your hand you will want to subtract additional outs from those main types as well.
It’s probably most important to count the number of nut outs and not just any out. This is because often times when the money goes in on a rainbow board, you will not be the only one with a wrap draw and some cards that give you a straight will give your opponent a better one.