Now and then people ask me what I think about learning several languages at the same time. I can only offer my grain of salt based on some experience and common sense, but here it is:

If you are a beginner in those languages, forget about it unless that's all you have to do in your life and you can draw on a bottomless reserve of energy. In my experience, you are far better off investing all your available time and energy into a given language, read, listen to radio or TV, peruse grammar books and so on. If you spread your energy between two or more languages, it'll take ages to reach a level where you can read, listen and converse in a rewarding way.

If, however, you already master all 4 languages well, then you are just entering the lifelong stage of improving your language skills in a given language. This you can do for as many languages as you like at the same time. If I were you I'd take an easy and rewarding language to start with - russian is close to polish and it could be a very useful language to know, french is close to english and it's a smart language to know, etc... Then once you master it well enough, it will give you more energy to go after another language. Don't forget that instead of spreading 3 months of study over 4 languages, you can use all this time for just one. If you just look at hours spent studying versus languages acquired, you'll be much better off concentrating, no questions about it.