I had a similar dilemma when I came here to Japan to begin at a Japanese language school. I’ll describe my situation and how I feel about it now almost 2 months into it.
When I came to Japan, I had a fairly solid N5 grammar when it came to reading and mostly with writing. But not up to that in speaking. I was WK level 10 and I could read all the kanjis of level 9 and below, I can also write them on a computer. Writing them physically from memory though? Nope. My vocabulary was mostly N5, except words that showed up on WK. Also far too many N5 kanji are in WK level 10 and I hadn’t yet gotten through the lessons for level 10.
School started with a test and two “beginner” classes were started. 1A and 1B. I was put in 1A (I think to get into 2A, I would have needed some real knowledge of N4). 1A’s first week of classes when it was still possible to move class if something didn’t feel right, were easy as heck. Barely above これはペンです。And I saw a future of being bored to tears for however long it would take us to get anywhere.
So I talked to the teachers. I learned that 1A would go through the first textbook, which was more or less N5, in 3 months. 1B would go through it in 1.5 months. So everyone in 1B had a mostly complete grasp of N5 but needed a bit brushing up before continuing on. While 1A had people like me who might have good skills in some areas but not all (grammar was good for me, speaking and listening pretty darn bad, and vocabulary so-so), it also had complete beginners who barely knew ひらがな.
I also looked at my goals for why I’m in Japan when I decided. In this case, I didn’t want to be forever behind and fighting to catch up. That would create a lot of stress, and I didn’t want that. I want to have time, guilt-free time, to explore Japan and have fun. Plus I want/need a part-time job, which also argues for not changing to the much faster class.
My teachers thought I should stick with 1A because of my speaking and listening ability was so low. I had decided to stay for the same reason.
Am I happy with my situation? Yes. I haven’t yet been bored through a whole class. Is it a bit boring when they go through a verb conjugation I already know super well. Yeah, sure it is. But is it boring all the time? Nope. And my speaking and listening skills are leaping forward because those are the parts I focus on the most. Secondarily I am firming down the vocabulary. I’m getting really good at the particles and when to use which one and not feeling as unsure.
I’d also already committed to a year of study at this school, and 1.5 month difference isn’t that much in my whole Japanese language journey. But I wouldn’t have picked this option if my school didn’t focus a lot on talking (and listening). Grammar is taught as sentence structures, not explained much (I have an advantage there, using BunPro and reading up on the grammar myself). But we are always talking, doing practice scenarios and I think we typically have one short presentation a week (not quite, maybe closer to every two weeks). Due to this I’ve managed to ask my teachers stuff outside class completely in Japanese. We had a field trip with school last week and I had a long conversation with another teacher at my school (it was partly long because I had to think about how to say things and he had to simplify, but still).
But you might have noticed how I at multiple points pointed out several personal reasons for certain things and how my specific circumstances formed my decision.
Are there a lot of speaking practice in your classes? Then it might be worth going for year 3 classes.
Are there more lectures and writing assignments and not much speaking? Then it is probably worth it to go for year 4.
Do you feel very comfortable with your own studies? Can you use what you’ve learned while speaking? Then year 4 seems better.
If you aren’t comfortable, but mostly okay, and using it in speech is hard and/or very slow. Then maybe year 3 is better.
How many other classes are you doing and how hard will they be? Maybe it is better to go with year 3 and cement that if you have a lot of other super challenging classes. The reverse is true too.
How about the rest of your life? Part-time job? Friends? Hobbies? Building a side-business? All these things affect your decision too.
I try to always make decisions by first looking at the small picture, in this cases my ability with Japanese and how the classes were going to be held and how fast they would go. Then I looked at my whole life, and thinking about how much time my different options would take and such.
I think I could have done 1B and done it well enough to advance at the end of term (aka pass). But I would have been struggling after a couple of weeks. I’d have been behind on vocabulary and speaking and listening would have been super hard and I might have fallen further behind because I couldn’t keep up with those things. Then when we continued on I would have kept being behind on those things.
In 1A right now instead, I am nailing down my knowledge of N5 grammar, really learning the particles so they are coming almost naturally. My speaking skill is growing well and so is my listening. I enjoy having only about 10 completely new words to me per chapter and perhaps 10ish familiar but not solid words. Instead of around 50 new vocabulary words per chapter.
But my situation isn’t yours. Hopefully, my reasoning and experience helps you make a decision, however I’m not going to presume to guess which one is best for you.