Format variable as 4 digits with leading zeroes

You would use -format operator:

'{0:d4}' -f $variable

https://ss64.com/ps/syntax-f-operator.html

the above will work if your variable is an integer, if not you can cast it to integer:

'{0:d4}' -f [int]$variable

Just to avoid having PetSerAl's useful help go to waste (should the comment be deleted at some point):

Aside from using the format operator (-f), which I would consider the preferred approach, you can also use formatting methods provided by the respective value.

• If the value is a string (as it seems to be in your case), you can pad it with zeroes:

  '26'.PadLeft(4, '0')
  

• If the value is numeric you can format it as a string:

  (26).ToString('0000')
  

Foreach versions of padleft and tostring. The 0 in the first one has to be quoted:

'4' | % padleft 4 '0'
0004

4 | % tostring 0000
0004

Working with a range:

1..10 | % tostring 0000

0001
0002
0003
0004
0005
0006
0007
0008
0009
0010

With a prefix:

1..10 | % tostring COMP0000

COMP0001
COMP0002
COMP0003
COMP0004
COMP0005
COMP0006
COMP0007
COMP0008
COMP0009
COMP0010