It will charge you the 4-5 fare, as you've already paid for Zones 1-3. Just ensure you touch in & out correctly and it should charge you correctly.
However if you are doing a mixed mode journey involving Zone 1, you can end up paying the premium (as discussed in other threads including yesterday!) even though the Zone 1 part is covered by your Travelcard, so depending on the journey you are doing, you may need to break an OSI by touching in at a bus (and immediately exiting the bus) or alternatively touching out/back in at a non-OSI.