Yes, you can easily switch between K and Q (and call functions defined in either language from the other [side of the same coin]).

K4 is a significantly more powerful language (tables are a first-class data type). Stevan Apter summarized the differences here:

http://groups.google.com/group/personal-kdbplus/msg/f18fb3a8...

Same ideas, very different implementation: * ksql was folded into the language; that means ksql datatypes become native (time, date, ...), and tables become first class citizen. This actually simplifies the whole thing considerably -- from a philosophical point of view, k4 has exactly 3 data types: atoms, lists, and dicts. Everything else is a trivial-in-retrospect application of these 3. * "electric" gui is gone; dependencies are still there in pretty much the same way, but triggers take a wholly different form. So, 2-line spreadsheet implementation is gone :( * The _verb form is gone

The language is essentially the same, even though your scripts will need porting due to these changes.