I agree. You shouldn't have to go with 4s -- however, when you DO go back to 5 after the effects of the antibiotic are done - you can still get to 5 by taking a 4 on one day and a 4 and a half of another 4 the next and still easily get to 10 each two days. With 4s, you may be able to more easily go to 6 until your INR is stable again, and go back to 5s.
With 5s plus 4s that you can split in half if necessary, you can easily get doses of 2, 2.5, 4, 5, 6, 7, 7.5, 8 - not that you'll ever need all those different doses - but having 4s gives you the ability to make more small changes if your INR is above or below range. Plus, your clinic will believe that you're actually listening to them and following their (probably flawed) recommendations.