Problem with selecting radio buttons based on previous choice

I have an issue with a PHP form where I have four separate, distinct sets of radio buttons. These are for people to sign up for certain workshop choices.

So, Workshop 1 has four radio button choices, and the user has to select their preference- 1, 2, 3 or 4.

Same for Workshop 2, Workshop 3 and Workshop 4, they each have four choices.

However, at the moment the user can choose a preference of 1 for the first workshop, then choose 1 again for the 2nd and so on. Most people are intelligent enough not to do this but some are not, so we need a way to prevent an option being selected in any of the remaining Workshop groups if that same option has already been selected in the first one.

So for example, if the user selected a Preference of 2 for Workshop 1, then the option / preference shouldn't be selectable for any of the following Workshops. They should only be able to select 1, 3 or 4.

Then, if say they selected a Preference of 3 for the next Workshop (Workshop 2) then the available options in the NEXT Workshop are limited to 1 and 4. And so on.

I can't see any way of doing this, as it needs to be done before any data is sent- does anyone know if this is possible and how? Thanks!