How to convert 'long' to four bytes in Bourne shell

[iamef]

Dear All,

Is there a way to convert long decimal int value to 4-bytes, using Bourne Shell or any commands from minimal Linux (sed, awk, etc)?
The goal is to calculate size of file and place the 4-byte value before it. I.e:

#!/bin/sh
size=`du filename`
cat <convert $size to 4 bytes> filename >filename

Appreciate your help.

UPD: i have found a part of solution:

n = 12345 # sample long int file size
"let n0 = $n % 256"
"let n = $n / 256"
"let n1 = $n % 256"
"let n = $n / 256"
"let n2 = $n % 256"
"let n = $n / 256"
"let n3 = $n % 256"
echo $n0 $n1 $n2 $n3

So now I have these 4 ints representing bytes. The remaining questions is how to write it as 4 bytes to file?

[sicarie]

The size of a long int will depend on several different things.

I would recommend checking the size of your long ints, it may already be 4 bytes.