base4dna.py

s = 'actgactgactggatc'

m = {
    'a': 0,
    'c': 1,
    't': 2,
    'g': 3
}

bytestring = []
for b in range(0, len(s), 4):
    byte = 0
    for p in range(0, 4):
        byte += (4**(3-p)) * m[s[b+p]]
    bytestring.append(byte)

print(bytes(bytestring))