There are many cryptographic techniques for doing anonymous voting. This is a well-studied topic.
I don't really recommend forum voting for sensitive topics, but if you really want to, you can do something like this:
Answer 1: Red
Answer 2: Green
Answer 3: Blue
Answer 4: Yellow
Click here. If 1, give your honest vote. If 2, click here and give the answer indicated regardless of what your true response would be.
As the number of respondents increases, the random noise evens out, and at infinity the actual percentages will be exactly the same as if this randomization stuff didn't exist. (Assuming that everyone actually follows the instructions.) You can use statistical techniques to exactly quantify this stuff. But yet each person has plausible deniability. Note that this scheme assumes that everything associated with random.org is perfectly secure, which may not actually be a safe assumption -- it'd be better to use dice or a local CSPRNG, but that's more difficult to get people to do.