Use the str.split() function with the maxsplit option to limit it to the first 3 commas. Then you can use a regular expression to extract the arrays from the rest.

import re

array_rx = re.compile(r'[(.*?)]')

with open("testarrays.txt") as f:
    for line in f:
        name, type, size, arrays = line.split(',', maxsplit=3)
        # split the arrays string into separate elements, removing [] delimiters
        arrays = array_rx.findall(arrays)
        print(name, type, size, arrays)

Without a regexp it can be done in the following way:

lines = [
    "dummy-test-foo,rsa,4096,[DNS.1:dummy-test,DNS.2:dummy-test-foo2],[IP.1:192.168.10.5,IP.2:192.168.20.5]",
    "dummy-test-jerry,rsa,4096,[DNS.1:dummy-testX,DNS.2:dummy-test-jerry],[IP.1:192.168.40.5,IP.2:192.168.50.5]",
    "dummy-test-tom,rsa,4096,[DNS.1:dummy-testY,DNS.2:dummy-test-tom],[IP.1:192.168.60.5,IP.2:192.168.70.5]",
]

def parse_line(line: str) -> dict:
    result: dict = {}

    # Split by comma
    line = line.split(",")

    # Find all words containing colon
    for word in line:
        if ":" not in word:
            continue

        # Remove brackets and convert to key / value pair
        word = "".join(char for char in word if char not in "[]")
        key, value = word.split(":")

        result[key] = value

    return result

for line_ in lines:
    parse_line(line=line_)