to avoid inconvenience, confusion, and a lot of potentially unnecessarily expended energy, frustration and lost time, opportunity and so on.... which is very possible...
I would book a highly rated hotel for the first two nights (plenty of great ones for less than 100AUD/night) ... and then you can slowly look about ubud and environs only switching if you want a different experience for the other two nights (for example, you might want to move to some place more outside of ubud or vice versa).
you want to spend your time shopping, dining and sightseeing not going from one hotel to another inspecting them, bargaining and so on. how many hotels could you feasibly really check out in a reasonable amount of time anyway? 2 , 3 , 4? most likely, you will just settle on the first or second one you see after discovering your tired, hungry or thirsty! :)
thats the murphys law of traveling in foreign countries ...imo
hope you enjoy!