What sort of CPU/GPU you running? If it can support 4k, then I'd say hold off so you can get a 4k monitor and utilize the new hardware to its fullest potential.
More and more 4k options will be coming into that price range sooner than we think. You can get a 4k monitor for $300 already -- a year ago that wasn't possible. With 4k quickly becoming "the new standard," manufacturers are going to make products that meet this market demand within your current price range. It's just not quite there yet, but will be soon -- hence my recommendation to wait and save up for a few months.