I would say you are experiencing the very common experience of confirmation bias. While every woman is different, most start showing no earlier than 12 weeks. That's because the uterus hasn't moved above the pubic bone until that point - so it's hidden behind a nice, thick bone which isn't going to suddenly pop out.
Other signs of course can indicate pregnancy, but so early on it's unlikely anyone can tell (maybe a dog?). Even home pregnancy tests can't tell that early. (I assume "2 weeks along" really means 4 weeks as a doctor would call it - around the date of your next period - as most conceptions occur around 2 weeks after the prior period, and your "weeks" count starting from the prior period). As such, I doubt your co-workers can really tell you're pregnant; unless you started acting differently (say, started wearing baggy A-line dresses all of a sudden), odds are they think you look pregnant because they know you are pregnant (or, because they know you are trying).
Your four year old will probably be able to tell you're acting a bit different, particularly as your hormonal changes start to hit and you change your eating and sleeping patterns, as well as if your moods change (which often, but not always, happens, often in multiple different ways). This latter is likely the best reason to tell him: if you find yourself more irritable, for example, it would be very helpful to tell him why, so he doesn't think it's his fault. But I don't think he'll know what it means even as you start to get your bump; he probably doesn't have much experience with pregnant women, after all, so he won't know what it means.
Other than that, you probably don't have to tell him particularly early on. I wouldn't wait until the due date, of course; he'll need a few months to get used to the idea of having a little [brother/sister]. In our case we told our two year old pretty early - around 20 weeks, if I remember right - and that seemed to work pretty well, but 2 is very different from 4.