s = '''col1, col2, col3, col4
5, 1, foo1, foo10
5, 2, foo2, foo20
6, 1, bar2, bar30
'''
res = {}
heads = ('col3', 'col4')

for line in s.splitlines():
    g1, g2, *vals = line.split(', ')
    res.setdefault(g1, {})[g2] = dict(zip(heads, vals))

print(res)