Price of iPhone in Japan

Hey everyone, this might be a long post, so please bear with me here...

I'm from Malaysia, currently using an iPhone 6s 64GB. I'm going to Japan at the end of the month for my undergraduate studies which is about 4 years.

I've heard from many people that iPhones are cheaper in Japan by quite a bit. So I'm thinking I want to get the new iPhone coming at the end of the year. I'll be cancelling my current SIM card because I don't think I'll be needing a phone number (SIM card) for now. Is that OK? But I can still use it for messages and calls using LINE or other apps as long as I have Wifi right? Or should I unlock my iPhone 6s before I go, just in case?

I also wanted to ask if anyone knew the cheapest option to get an iPhone in Japan? I'm guessing buying from providers like docomo and AU through a 2-year contract or something like that? Can I do that if I wanted to pre-order the new iPhone in September? What kind of price should I be expecting in terms of total price and monthly payment amount?

TL:DR - Can I live in Japan without a phone number for a few months? What's the cheapest option for buying the new iPhone 7s/8 at the end of the year? Approximately how much would it be in terms of total and monthly payment amount?

Thanks! ( I can read and speak Japanese if that makes any difference )

Yes, you need a phone number. Pretty much every form you will fill will ask for one, at school, at the city office, at your bank, at the doctor's, when buying a commuter pass for trains/buses, when buying something online, and so on.

Y mobile will be the cheapest option for 3000 to 4000 yen +- per month.
Docomo and Softbank same contract but double the price

I guess I'll need to get a SIM card then... Any recommendations?

The 3k to 4k+ yen per month is for a 2 year contract right?

You could get a VOIP number. There's a few ways to go about this, but the only one I remember off the top of my head is Skype. You can get a Japanese number on a month to month basis, and it should be a fair bit cheaper than full cell service.

You can opt for the cheaper option with MVNO carrier, where they lease off the main provider. Price start from 1,*** yen depend on data package(min. 3GB) with phone number.
Google OCN Mobile, BMobile, Iijmo, UQMObile, mineo etc.
If you can read Japanese, explain, list and compare available MVNO.
http://kakuyasu-sim.jp/voice

Or since you going as student with 4years, you can signup with any of the major 3 telco for student package which will be much cheaper with student discount.