Since you bring it up and I don't mean to boast but if you think 4 is special:
I speak English, German, Italian, Russian, Mandarin, Minnan, and Wu, in addition to reading ability in French, Spanish, Portuguese, Dutch, Danish, Norwegian, Swedish, Polish, Czech, Croatian, Serbian, Bulgarian, Slovenian, Ukrainian, Vietnamese, Korean, Japanese, Malay, and Indonesian. In addition to those I have basic ability in Cantonese, Hakka, Arabic, Mongolian, Uyghur, Turkish, Hungarian, and Hindi.
If you add them up, that's 7 languages I speak, 19 more I comprehend, and 8 more with basic abilities. That's 34, so, 3 or 4 languages in comparison is nothing special.
Those 19 languages of comprehension would only require about 3 or 4 weeks in that location for me to acquire a good speaking ability, since I have already studied much of their related vocabulary and grammar. So if I had the time and money to do about 20 months of globe-trotting in the right spots with the right environment and regimen, I would be able to speak 26 languages within 2 years. I don't find it very difficult if you have your own system of categorizing vocabulary and a good memory.
I also claim that I can learn an unrelated language I've never touched before in 5-6 weeks as long as I have full-time available, and contact with a native speaker.
So to answer the question WHY? Because it's fun and a very pleasant experience that opens you up to new cultures and peoples. For somebody like me, it's great because I'm in constant contact with multinationals every week.
Hey, Mark, how about yourself? I was under the impression that you know quite a few yourself, including the Chinese ones. Kannst du auch einige Europäische Sprachen? Hoantsai, goa ma guanyi ka li sia banlam-oe e phoe, tse si kong, nasi li u chhubi, tioh ka goa lianlok. Goa chin hoaNhi u chit-e kihoe ka li anne liansip!