As far as I understand, you are trying to overlay 4 pages on each A4 portrait, which will give you 4 A6 pages (two A5 sheets for an A4 page), since dividing the A4 area into 4 parts means dividing each side into two parts, so we will have :
29.7 cm / 2 = 14.8 cm (approximately)
21 cm divided by two = 10.5 cm

it is obvious that in the same area of โโthe A4 book format you can find a place, 4 single-page A6 pages and two A5-pages A5, as in the picture:

So, you donโt want to make an A6 booklet, but only to cut the A4 portrait by half to create 2 A5 sheets for an A4 page? Are you not interested in shortening these A5 landscape pages to link the A6 booklet?
Since, as I understand it, it seems to you that you are looking for a script that can create a booklet overlaying 4 A6 pages for each side of A4 sheet, recounting the overlay sequence in the correct order to have the right front / back match, take a look at my script
note: before starting, resize to A6 to your input multi-page pdf (A4 size)
this can be done with
Multivalent.jar
java -cp path...to/Multivalent.jar tool.pdf.Impose -dim 1x1 -paper A6 input.pdf
then take the resulting output file (don't forget to work with the copy) rename it to the original input file and use this script
to accomplish this task and to ensure that your multi-page pdf file has the right number of pages needed to perform the overlay (8 or an integer multiple of 8), you will use this script based on pdftk (at less than 1.41 version with print and Multivalent.jar - - http://ge.tt/6fVll5H (latest free version with tools included).
use: namescript file.pdf multi-valued path
the ambiguous path is relative : if you downloaded Multivalent.jar to your home, the ambiguous path will be / home / . You can also configure the script and immediately replace the value for the second argument (multivalent path, so you do not need to enter the multivalent path every time, in this case replace multivalentpath=$2 with
multivalentpath=/home/Multivalent.jar
the script will also add crop marks to the final pdf output
viewing the animation or the resulting output file with the added crop marks to simplify cutting twice the number of sheets (first horizontally and vertically)

as soon as you finish typing and place the last sheet on other sheets, look at your last superimposed sheet

this need to be cut off (I selected with a dashed line) halfway horizontally to close two series of sheets (up and down) with each other (upper part lower), getting a single series of sheets from two original rows of sheets,

at this stage, you can additionally cut the sheet along the dotted line to be able to match the individual sheets of A6 and expecting with the help of glue, a clip or another, or using two points of a stapler, the bend of the booklet that you created
Note: the blank pages that you saw are automatically placed after the last written page of your original pdf, they are necessary, as you correctly said in your question, to perform the correct overlay sequence, they do not interfere with the logical sequence in the resulting booklet-bound order, in our example would be:
1,2,3,4,5,6,7,8,9, 10,11,1,2,13,14,15,16
pages in bold will be left blank and necessary only for proper overlay, which cannot be done if the number of pages is not a multiple of 8