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SPI4 signals are available as default functions only on the 337BGA package. No multiplexing configuration is necessary to use these dedicated terminals as SPI4 signals.
On the 144QFP package, SPI4 signals are only available as alternate functions on assigned terminals. This requires multiplexing control registers' configuration. Configuration is also required for functional inputs, as this controls aspects of the I/O buffer such as pull enable/disable and pull direction.
Consider the N2HET1[5] / SPI4SOMI / N2HET2[12] on pin 31. The function is selected using PINMMR5[16, 17, 18]. By default PINMMR5[16] is set causign N2HET1[5] to be the function selected. This means that all aspects about the I/O buffer on pin 31 are controlled by N2HET1. For example, N2HET1 terminals are pulled down by default, by virtue of the reset value of the HETPSL register.
For a SPISOMI function, you typically require a pull-up on this input function. So it is better to choose the SPI4SOMI function on this pin 31 to get the pulls (and other I/O aspects) controlled by SPI4 registers.
