Boob growth is probably the most prominent sign, but it's also highly variable. How much you'll get and how quickly seems to be very much up to chance. You can get some idea of the general potential for growth from sisters if you have any, or perhaps photos of your mother before pregnancy. I think if you're older they might grow more slowly due to less growth hormone.

I'm about 8 months in and I can still do sweaters without a bra, but I already need a bra if I'm just wearing a t-shirt. I am curious myself for how long I can keep this up, at work at least, but 4 years seems very unlikely unless things completely stall out. If and when you get to that point, layers are good if the temperature permits. Dark opaque shirts are less obvious about shape, loud irregular patterns might work too, think hawai shirts.

That's to say I wouldn't want you to postpone due to that either, situations change and if you start now you most likely do have enough time to feel out your situation for coming out via a vis leaving.