Complex form logic based on earlier selection

[JamesWB]

I have a situation where a selection is made in a form dropdown between, firstly, 4 different values - depending on which is selected, this loads the next dropdown which has three more.

Once those are selected, I then load one of four forms which need to have different fields showing based on the second dropdown.

I don't think this is a dependent dropdowns problem, it's that I want to either 'grey out' or remove certain fields from the form that is displayed after the second selection.

How would I go about doing this in general terms please?

thanks

James