connecting 4 GPIO pins to DAC using SPI interface

The connections are correct. This way your Nios processor can directly write the spi master control port to transmit data.

Anyway if you really want a 1Mhz data rate, I believe you'd hardly achieve it with Nios operating this way. Nios is not a very fast processor and if you want to use it for other tasks in your design, it would not keep up with that transmission rate.

You'd better use a dma to transfer data from your memory to spi.

Regarding clock rate, since the DAC requires 18bit per data (including start and stop bits), you need at least 18MHz. Then, for example, if your system clock is 100MHz you can set SPI sclk to 20MHz.