The K4 is the replacement for the K3S + P3. This is a long-planned transition, and we've gradually drawn down stocks of the latter, including all options.
I'm sorry we can't provide options for all of our legacy products indefinitely. But we will of course continue to repair and otherwise support them.
We have three complete transceiver product lines aside from the K3S: The ultraportable KX2, its larger brother the KX3 (used by many hams as a compact home station), and the K4, our first hybrid-SDR desktop radio (with matching KPA1500 amp). All are very much active and will be for the foreseeable future.