Hi,
First of all thanks for the question.
Before I try answering your query, I would love to know your level in 3BLD.
First things first, the entire word does audio as letter pairs with small words and phonetics created out of them. For example BG, becomes BaG or BiG. The vowels to add in between depends on which word or sound you want to make and which ones you are more comfortable with during audio memo.
So TL;DR, always vowels between letter pairs to do audio memo.
Now coming to 4 letters which is my speciality and what I use for memorization for long term memorization as well. Letter quads is not easy to make and it is a lot of hard work. I would not recommend that you even explore this idea for a few years let alone give it a shot.
Here is the documentation I made on LQs in 2019, you can have a look at it.
Letter Quads Documentation: How to memorize any worldly information in 4 letters? Personal Introduction: Am I a linguistic or a wannabe memory athlete? This has been my linguistic endeavor since I first made my letter pair list way back in 2013. I remember that I had chosen 450 random im...
docs.google.com