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The SPI block in PSoC can support 64 clock cycles transaction with each data width as 16 bits. If you need 4 MISO inputs you may have to create a custom component as you rightly said. Please note that the SPI master of PSoC can operate at a maximum of 25 Mbps (SCK). Choose a PSoC 6 part that has UDBs in it so that you can implement your custom component in hardware.
PSoC® Creator™ Component Author Guide
Universal Digital Block (UDB) Editor Guide