Disabling Reprompt Button

[anoop_ind2]

I have 4 prompts in the page.  Each one is a cascading prompt. ie. 2 is  based on 1 and then 3 is based on 2 and 4 is based on 3. Prompts 2/3/4 are multiselect. Because of this reason i added a Reprompt button in my page.
Now after selection of all the 4 prompts i do not want the Reprompt button enabled.  i.e once the 4th prompt value is selected the Reprompt button should be disabled/greyed. Only Finish button should be enabled. How do i go about with Javascript for this? or is there any other way to do the same?

Thanks in Advance
Anoop S