I also use my i4s at work daily. The fact that they are semi open is actually a huge plus for me. It means I can hear if someone calls my name or needs to speak with me. As for leakage, this has been addressed several times already, but briefly, while there is some sound leakage, at normal listening levels it is pretty minimal...Unless your office is library quiet and you have coworkers sitting right next to you, sound leakage should not be loud enough for your coworkers to hear. It is not loud enough to hear across a cube wall, for example, at the levels I listen to music. If I take my i4s off and put block the tips with my fingers (as your ear would when they are on) I can barely hear any music is playing from more than about 3 feet away....and that's in my own office that I don't share so it is a fairly quiet room.
As for how quiet you need the room to fully appreciate the sound of the i4s, having some ambient noise, souch as a typical office environment, does not, for me, detract from the sound quality. Listening to them in my office, I still routinely hear details they resolve in music I know extremely well that I have never heard with other headphones or iems, include very highly resolving and isolating ones like my Noble encores. Background noises will definitely intrude if you try to use them in loud settings like public transport, and you would be silly to use them on a plane. But at least for me, the i4s are so enveloping/engaging that low-grade background noises like a coworker walking by or an office printer down the hall don't suddenly ruin the sound. Do you, in an absolute sense, lose some resolution if you aren't in a totally quiet space? Yes, but unless you are really listening very analytically or are listening at very low volume, low grade background noises do not put a damper on my enjoyment of the i4s.