Is it reasonable to ask update on recommendations letter twice?
You can stop worrying.

It is not unusual for professors to wait until the last minute to write letters.  Although busy, they are experienced at meeting deadlines and will get it done.  It only really takes an hour or two to write a recommendation letter, and submitting it to 7-8 places is no big deal at all.  What you've asked of them is completely reasonable.

It is certainly fine to contact them, though I don't think I would phrase it as "asking for an update".  I would write an email like:


  Dear Professor X,
  
  Thanks again for agreeing to write me a recommendation letter.  I just wanted to send a friendly reminder that the first deadline is April 1 (for University Y).  The remaining schools and dates are in the list I sent you previously.  
  
  Please let me know if you find that there is anything else you need.  
  
  Sincerely,
  
  truvaking


Also, if you have recently made any significant accomplishments that you think they might want to mention in the letter, you can communicate this in your email as well.

I really don't think you will upset anyone by doing this.  It would be incredibly petty and unprofessional for a professor to go back on a promise to write a recommendation letter, solely because the student sent some reminder emails.  Likewise, I think it is still fine to re-contact the fourth professor if you think their letter would be helpful.  They may have simply overlooked your original request (professors get a lot of email).