1. intro must be imperfect:
"Обычно я не бронирую номер в гостинице,..."
Prefect form just doesn't have sense here. "Usually..." implies several times or continuous process, that is - imperfect. "Забронировать" is perfect form of imperfect "бронировать".
2. you should use imperfect (in both cases) here too. reason is the same - you said "usually" and this implies several times of "unusual things". "usually ... but sometimes ..." is implied here. Note that all phrase is set in something like "present indefinite" of russian (we can say "present impefect" or just "present" because there is no present for perfect in russian):
"Обычно я не бронирую номер в гостинице, но если это случается я беру номер в гостинице с тремя или четырьмя звездами."
note that you can use perfect verbs in the second part if you will say "I never..." in first part. This will change things. As far as second part never happened before you should switch it to perfect verbs, meaning first possible occurence of event. You can keep it imperfect, but as far as this never happened before you must use future form which is more complex: "но если такое будет случаться, то я буду брать номер в гостинице с тремя или четырьмя звездами" - sounds overcomplicated a little.
If you omit "обычно" - "Я не бронирую номер в гостиниах..." it becomes similar to case with "never". Also if you add "...но если это снова случится..." you can use (imperfect)+(perfect+perfect), because you make clear that you mean next possible occurence of event.
This complexity makes your variant sounds almost normal because it is normal in russian to omit words. I think many natives can accept it without noticing problems of sync between first and second parts in fluent speech.