getting accomadation in Japan

i've looked into the hostels and hotels that are recommended and would like to know if it is better to book in advance or if i would be able to get a room (not fussed about condition) on the day? i've never been travelling properly before and never to Japan, im looking at about 4000-5000 yen per night.

Most hotels will accept same-day bookings, IF they have rooms available. Some ryokan and minshuku which have meals included, especially smaller ones, might not be able/willing to accept same-day reservations.

And, of course, there is a risk that hotels are booked out.

Even if you prefer to travel flexibly, I recommend to make reservations at least the day before. And if you don't need flexibility, I see no reasons for not making reservations longer in advance.