text = '''"Value1", "Value2", "This is a longer piece
of text with
newlines in it.", "Value3"
"Value4", "Value5", "Another value", "value6"'''
import re
pattern = re.compile(r'".*?"', re.DOTALL)
print pattern.sub(lambda x: x.group().replace('\n', ''), text)
dGV4dCA9ICcnJyJWYWx1ZTEiLCAiVmFsdWUyIiwgIlRoaXMgaXMgYSBsb25nZXIgcGllY2UKICAgIG9mIHRleHQgd2l0aAogICAgbmV3bGluZXMgaW4gaXQuIiwgIlZhbHVlMyIKIlZhbHVlNCIsICJWYWx1ZTUiLCAiQW5vdGhlciB2YWx1ZSIsICJ2YWx1ZTYiJycnCgppbXBvcnQgcmUKcGF0dGVybiA9IHJlLmNvbXBpbGUociciLio/IicsIHJlLkRPVEFMTCkKcHJpbnQgcGF0dGVybi5zdWIobGFtYmRhIHg6IHguZ3JvdXAoKS5yZXBsYWNlKCdcbicsICcnKSwgdGV4dCkK