How to make next button appear when out of 6 buttons only 4 buttons are clicked.

I use a shared action for all comparable situations with a menu slide. 

Summary:

1.  Create a user variable for each of the buttons, will label them v_1, v_2.....v_6. Start value =0
2. Add another user variable v_clicks, also with start value = 0. It will be used as counter.
3.  Add the Next button and hide it in Output, either using the Properties panel or with the On Enter action.
4. Create this shared action, to be triggered by the buttons:
       Decision 1 'Always'  non-conditional
          Increment v_1
          Jump to chapter 1
       Decision 2 'Counting'   conditional
           IF v_1 is equal to 1
                Increment v_clicks
         Decision 3 'ShowNext'
           IF v_clicks is equal to 4
                Show Bt_Next

The items marked bold and italic are the parameters: variable v_x, slide Chapter1, button Bt_Next. Eventually you can also mark the literal 4 as parameter if you want to use the action in future projects where the number of buttons to be clicked can be different.

If you use the On Enter action to hide the Next button, you could move the third decision to that On Enter action:

IF v_clicks is equal to 4

   Show Bt_Next

  ELSE

    Hide Bt_Next