Here's what I will probably do:
SGD
250 - G-shock Mudman which I bought for $190: Something to use for Army, and physical training. The additional functions of compass and temperature are useful too. No matter how much of a WIS you are, you can find a use for a G Shock!
500 - Hmm... Nothing much really jumps to mind. I'll probably get the CW C8 Pilot Vintage Edition. It is (or maybe offer over? Not sure) going for 50% off at US$380+. I also just bought a Gruppo Gamma Vanguard PVD for $470 if you're a Panerai fan.
1K - Steinhart OT500, got mine for $900 thereabout with GST.
2K - Seiko MM300, recently there was one BNIB sold in Carousell for $2k, can probably get under $2K if you buy in Japan. 2nd choice will be Oris 65 blue face, but have to work hard to find one at this price range. Some said it's small, but I love that size to wear as a dress watch.
4K- This is the hardest for me because I have not considered buying any watch priced at 4k before so.... but if you give me 4k to spend, I'll buy the Tudor BBB and pocket the change.