hardiebacker thickness

[Dr.Lee]

Our local big box store has hardiebacker in 1/4" only. I purchased 2 sheets for my project for the top of the block stand. I did not learn until now that there is also 1/2" I have seen posts where people are using 1/4 and others using 1/2. Do I need to go back and get two more 1/4" sheets before the concrete pour for the form happens? I should add that I am building a 39" oven and that the form support per the description and photo on page 26 of the plans is in an opening of 47" x 62". I am going with a 3.5" concrete pour since I am using FB board.

UPDATE: I purchased 1/2 and made a switch.
Thanks!

[Bentham7]

I don't have better experience of using this hardiebacker thickness. But I would like to get some more details about it.