OK, found them (below). So I can confirm that I use the correct pins (PA12 = MOSI, PA13 = MISO, PA14 = CS, PA15 = SCK, NINA_GPIO0 = PA27, NINA_RESETN = PB08) on the correct sercom (4).
What is strange are still the pins logged by Arduino IDE (they clearly correspond to SPI0 - and the sketch works with NINA!). The program is still stuck while waiting for NINA_ACK to be high...