About six weeks ago, I started my voyage into a fourth language.
English is my native language.
I learned Japanese fairly well years ago but hadn’t studied it at all or much practiced it much over the years. I’ve retained the grammar but the recall of vocabulary is quite impacted by the years. Further, I doubt I’d be able to write too much, at least by hand. That said, I’m very comfortable conversationally and was at the equivalent of a B2 level back in the day.
French has been my primary language of study for the last several years. I’m at a B2 level here.
(I also have Mexican restaurant fluency but no other skills in Spanish 
)
In August, I started a journey into German.
First, I’ve noticed what seems to be my brain’s improved ability both to compartmentalize each language and to be less taxed by language study in general.
Second, I’ve noticed that I’m drawn to pick back up Japanese study as I proceed ahead into German. Somehow, it almost seems as if my brain refreshes its in-the-background language interactions, it’ll help me subconsciously with my brain’s grappling with a new language. I’ve been doing a little bit in Japanese every day for the last few weeks.
Third, the similar-but-different aspects seem the hardest for my brain to wrestle with. For instance, English and Japanese don’t have gender, but German and French do but it works somewhat differently than, say, maybe the higher consistency encountered by a learner of multiple Romance languages.
Would welcome thoughts and tips from persons who have relative fluency with four-plus languages, especially native English speakers that have taken up a) a Romance language, b) a Germanic language, and c) something else very different. English has such unique relationships with German and French that I wonder if that’s substantially related to what my brain is going to get to deal with in what’s ahead.
My German goal is just to get up to a A2 to B1 level in a couple years to gracefully interact with extended German family.
Thoughts and tips?