I have been self-studying for some months now with Minna no nihongo. I am finishing now the first book, and as Noken exams are coming next July, I am having a hard time deciding which level I should take. I mean, I think I can finish Minna no nihongo 2 in a few months, seeing how many hours I spend normally studying Japanese, but I don't know N4 is worth the risk now. (I don't use MNNH only, I'm also revising with Genki I and Speed Master N5)
Hello, I'd suggest sourcing some mock tests(which are available online for free) and having a go at N5 and N4 and see how you fare currently. If you are acing N5 already and still have time (some months, as you said) before the test, then why not go for N4 which will no doubt motivate extra study until the test date. If however you find that you're still needing some extra revision to easily get all the N5 answers and/or if the easier option? I see that your question was now 5 months ago so perhaps you've already taken the test... in which case, how did you fare? :-)
I found it easy to skip N5 and go to N4. The main reason is that once you get the gist of N5, it's incredibly simple. I recommend saving the 7500 yen and taking more time to study N4 is necessary.