I cannot think of a way to do this directly...but you could set course completion criteria requiring completion of the Cert 1 course before they can get credit in Cert 2. This will not prevent them from completing Cert 2, just getting 'completion' using the course completion criteria, if you use that.
Or...you could put all eight topics in one course, each with a separate certificate activity that issues when the four required topic items are completed. Make the second set of topics invisible unless the first four were completed.
Or...you could put all eight topics in one course, each with a separate certificate activity that issues when the four required topic items are completed. Make the second set of topics invisible unless the first four were completed.