Long and the short of it, I would keep your K3, I just got one myself, and not worry about the K4. First, Kindles seem to hold their price pretty well. K2s are selling for $100, sometimes more. Second, why wait based on the possibility of enjoying something when you could be enjoying it now. Third, if the K4 is released during the summer you will probably want to wait a bit any way so that the glitches can be worked out.
I decided to get a K3 because a smaller device is nice while traveling. I love my DXG in the house but it was a bit big on the plane. (shrugs). I kept a K1 up until the DXG was announced but I had been wanting a DX for a while. I was waiting for a price drop and the new screen and collections were wonderful bonuses. I have no idea what features they might add for a K4 but the K3 does what I need it to do and does it well.
I would be surprised if a new device was announced this summer. The K3 is still new on the market and there is no way of knowing how the earthquake in Japan could be effecting parts for the Kindle.
We know that car companies have seen a reduction in available parts for cars after the quake. I know friends in the film industry who have been talking about problem getting video tape and electronic parts for cameras after the quake. It stands to reason that there might be a similar problem for ereaders and other electronic devices.
Even if plants were not damaged by the quake of tsunami that followed there are problems with delivering electricty because of the reactor crisis.