How I can multiply the fourth one from each row in the text file

[Larz60+]

As philia states, it's unclear as to what your intention is, however to access the various elements of your file,
split each line into a list, then it will be easy to work with. Example:

import os

# make sure we are in same directory as script:
os.chdir(os.path.abspath(os.path.dirname(__file__)))


with open('fin.txt', 'r') as fp:
    linecount = 1
    for line in fp:
        line = line.strip().split(',')
        print()
        for n, item in enumerate(line):
            print('item {} of line {} is equal to {}'.format(n, linecount, item))
        linecount += 1

will generate:

Output:
item 0 of line 1 is equal to - 2
item 1 of line 1 is equal to  1
item 2 of line 1 is equal to  2
item 3 of line 1 is equal to  3
item 4 of line 1 is equal to  **4**
item 5 of line 1 is equal to  15
item 6 of line 1 is equal to  14
item 7 of line 1 is equal to  29
item 8 of line 1 is equal to  64168.0
item 9 of line 1 is equal to  1

item 0 of line 2 is equal to 2
item 1 of line 2 is equal to  1
item 2 of line 2 is equal to  2
item 3 of line 2 is equal to  3
item 4 of line 2 is equal to  **6**
item 5 of line 2 is equal to  15
item 6 of line 2 is equal to  14
item 7 of line 2 is equal to  29
item 8 of line 2 is equal to  64168.0
item 9 of line 2 is equal to  1

item 0 of line 3 is equal to 2
item 1 of line 3 is equal to  1
item 2 of line 3 is equal to  2
item 3 of line 3 is equal to  3
item 4 of line 3 is equal to  **8**
item 5 of line 3 is equal to  15
item 6 of line 3 is equal to  14
item 7 of line 3 is equal to  29
item 8 of line 3 is equal to  64168.0
item 9 of line 3 is equal to  1