Create multiple modified versions of a script for submission to cluster

If I am able to correctly guess what you want, your pseudocode was actually pretty close to valid Python code.

I am assuming model.mo is actually a text file, and that "non-.txt" in your question just refers to the .mo extension. Python (or Slurm, or presumably Modelica) doesn't care what you call your files; the extension is purely a human convention on sane platforms. (I am consciously wording this so as to exclude Windows.)

with open('model.mo') as modelica:
    script = modelica.read()
# did you really want a space instead of an underscore in 'control 4'?
for i in ['control_1', 'control_2', 'control_3', 'control 4']:
    ivar = script.replace(
        'moduleName = "control"',
        # guessing you wanted to keep the double quotes around the name here
        # this generates "control_control_1"; maybe you meant to omit the control_ prefix?
        f'moduleName = "control_{i}"')
    for j in [3, 7]:
        jvar = ivar.replace(
            '.CombiTimeTable solar_data(columns = {2}',
            f'.CombiTimeTable solar_data(columns = {{{j}}}')
        for k in [2000000000, 4000000000, 6000000000]:
            kvar = jvar.replace(
                'Storage.Battery BESS(EMax = 2000000000, SOC_start = 0.5, pf = 0.9)',
                f'Storage.Battery BESS(EMax = {k}, SOC_start = 0.5, pf = 0.9)')
            for l in ['4', '8']:
                lvar = kvar.replace(
                    '.CombiTimeTable ev_data(columns = {2}',
                    # guessing you meant to keep the braces, and you wanted l, not i?
                    f'.CombiTimeTable ev_data(columns = {{{l}}}')
                # Guessing as to what exactly the file name should be
                with open(f'child_model_{i}_{j}_{k}_{l}.mo', 'w') as child:
                    child.write(lvar)

In particular, perhaps notice how f'strings' allow you to interpolate variables with {variable}. The notation {{ inserts a literal opening brace, and }} a single literal closing brace. There are many options for how to format interpolated variables and expressions; for example {j:02} would produce the value of j with at least two digits and padding with leading zeros if necessary.

As such, your + str(j) + attempts would work fine as well; it's just a lot more clunky.

I added comments where I had to guess what you really wanted.

Nominally, using round parentheses instead of square brackets around the enumerations would be very slightly more efficient, but that's hardly a major concern here. My main concern would be whether the names of the generated files are what you wanted. If you want to abbreviate, perhaps use a dictionary, like

abbrev_k = {
    "2B": 2_000_000_000,
    "4B": 4_000_000_000,
    "6B": 6_000_000_000
}
...
        for k in abbrev_k.keys():
            kvar = jvar.replace(
                'Storage.Battery BESS(EMax = 2000000000, SOC_start = 0.5, pf = 0.9)',
                f'Storage.Battery BESS(EMax = {abbrev_k[k]}, SOC_start = 0.5, pf = 0.9)'

to use the abbreviation 2B in the file name, but have it translated to the numeric value 2,000,000,000 in the generated file.

Demo: https://ideone.com/cP7KRx

You'll notice that we use script to hold the contents of the original script, and then in the first substitution, we create a copy of it in ivar with the control string modified, then continue to create new variables jvar, kvar, lvar with a modified copy throughout the rest of the script.