Hi from Germany

Welcome, Siena! Your situation sounds tricky with so many languages, but very nice too. I, for one, am very envious, in the best possible way. I only have experience with 2 languages, so I wouldn't know how to help you. But I wanted to share a bit of my experience, in case it helps:

- You don't have to make the right choice today. There's room for change. You can choose a strategy today and change it tomorrow if you see that one of his 4 languages is lacking or needs more attention.

- Years ago, in your situation, I would have spoken language 1, the dad language 2, the dad and I would have spoken language 3 with each other and let the world around us take care of language 4. With my husband and I speaking in the ml with my daughter since she was born and the ML between ourselves, my daughter learned to ignore the ML and only spoke the ml until she was 3. So I like your strategy and how you're not letting your son only be passively exposed to English. I just wonder how you'll meet the "need" part of the equation with Farsi and Croatian when you sometimes speak English with him. I think you'll have to be very careful to make sure that your son doesn't reply in English when it's Farsi or Croatian time.

But, like I said, I'm just thinking out loud as I don't have any experience with multiple languages. Best of luck, Siena! Please keep us posted.

Siena, welcome! Yes, there are many "moving parts" to your situation, but it sounds like you and your husband have the right kind of proactive mindset that will enable your family to experience a lot of rewarding success on your multilingual journey together.

Let me stress, first of all, that there isn't one "right way" to pursue your multilingual aim for your son. That said, I would lean toward your idea above by emphasizing Croatian and Farsi with him, particularly during these first few formative years. After all, since it sounds like he'll be attending a bilingual school, with both German and English input, I expect he'll eventually acquire these two languages just fine without you actively using either of them with him.

In fact, the "danger" in communicating to him in English (or German) is that, yes, it could end up undermining his need to respond to you in Croatian and Farsi. To my mind, it's better to err on the side of caution and only address him directly in Croatian (you) and Farsi (your husband). In this way, you'll not only be providing him with more input in these languages (because using English with him will subtract from that input), you'll more effectively "condition" him to respond to you in Croatian, and your husband in Farsi, once the little guy begins to communicate more actively.

In my view, that's the first and biggest aim for your multilingual journey: "conditioning" him to use these languages actively by providing him with ample exposure in Croatian and Farsi, while nurturing a genuine need for these two languages, based on the parent-child bond. Once this first aim is initially accomplished, by the third or fourth year, it may be possible to use some English with him at that point, if this seems like an effective choice at that time. But you can cross that bridge when you get to it. 

In other words, I recommend focusing your efforts on Croatian and Farsi for the first few years. I don't think his eventual acquisition of English and German is a concern, really, and you'll likely experience greater long-term success by giving your two main minority languages a good "head start." On that note, since it sounds like your husband isn't able to spend as much time with your son as you are, which means there will be less exposure to Farsi, you may need to consider strategies for strengthening this input, like bringing in other speakers of the language, locally or online.

And finally, I would also encourage both you and your husband to make some regular effort to improve your own knowledge (even passive ability) of each other's languages. This will happen naturally over time, as you hear each other speak these languages to your son, but the more language ability you both can acquire, the more helpful this knowledge can be to your journey as it proceeds into the future.

Siena, I hope this initial response is helpful to you! And I hope my books can be helpful, too! Thank you for reading them!