RFPA2026 lack of info.

I read RFPA2026 datasheet, but there is not really a lot of info. in it.
There's no word about input and output impedance. I guess it is 50ohm?
No s parameters...

Only thing about matching is:
-Flexible External Matching for Band Selection

Any idea?

The datashseet shows suggested matching networks for different frequency bands together with s-parameter magnitudes, which can be read as an implicite impedance specification. It should be also considered that s-parameter specifications of power amplifiers are of limited worth because the impedances are actually level dependant.

As in any real RF hardware development, you'll start with the suggested matching networks and adjust it if necessary based on own measurements.

What is the purpose of L4(3nH) in last schematic in RFPA datasheet?
Thanks

In case of doubt, impedance matching. The values seems quite large at first sight, without own measurements you can just believe that it's right. It's not evident why the 2.1 GHz circuit uses a lower inductance value, suggests a somehow irregular S11 frequency characteristic.

One more question. Is the input and output impedance od those three stages 50 ohms or not? Is it possible to use just 50ohm PCB track without any matching? Exept AC coupling capatior ofcourse.

The LC networks are required to achieve 50 ohm matching. Impedance respectively S11 of the "naked" in- and output pins isn't specified. You may want to calculate a impedance from the matching networks and determine the mismatch when directly connecting the chip to 50 ohms.