I’m in a similar boat. I got to level 4 on Oct 27, and didn’t have the cashflow to buy a lifetime membership before the Christmas sale. I emailed them proposing to do some small work for a discount. (The lifetime 'ship is the only thing that really makes sense for me, since I know I’m going all the way to the end – and, perhaps more importantly, will be using it as a reference resource even long after I’ve finished – and cash will always be tight, so best long-term was for the cheapest overall price.)
They kindly and politely declined my offer (they generally don’t do such offers as I was making) and instead offered me a coupon for a discount for 12 months, and if I ended up buying the Christmas lifetime deal, they would pro-rate any extra I had spent on the 12-months into the cost of the lifetime. It was a very generous and courteous offer.
In the end, I was still hesitant, because I wasn’t sure what my finances would look like in the coming couple of months.
Finally, once I knew what my budget was for Christmas-time (which was just before Christmas, actually), I decided to just go straight for the lifetime, since the sale had already started by that point. In the meantime, I had had to ‘pause’ for about a month, which wasn’t so bad actually.
So, I actually decided to use a small low-interest loan (line of credit I still had active from years ago), to cover the ‘bulk’ of the up-front life-time payment. For me, I had spent quite a while considering different options, and this turned out to make the most sense for me, even though I’m very averse to using loans or credit or anything like that generally. But the timing, the fact that I wanted to get back to learning again without too much longer of a ‘gap’, etc. Even though I was technically going ‘over my budget’ in the short-term, in the long-term the investment was worth it to me. I’m greatly enjoying proceeding happily along in my Wanikani journey through the levels. I can handle some short-term budget-pain to squeeze this purchase in.
(I also purchased a similar life-time membership at BunPro.jp for learning grammar, and I’m equally glad I did that as well. Basically identical circumstances. The two subscriptions were my Christmas gift to myself. )
So, while I wouldn’t use my own anecdote to suggest any specific plan for anyone else, at least hopefully it will provide an additional perspective from another person for you or anyone to consider. I’m happy with my purchase. YMMV.