Max SPI data rate supported
In SPI each bit is transmitted on a clock edge, therefore the data rate (in bits) is the same as the clock frequency. Your datasheet is not providing the maximum frequency, but it is providing the minimum cycle time, which is \$250ns\$. As we know the relation between frequency and period time is \$f=1/T\$, we can calculate the maximum frequency as
$$ f_{max}=\frac{1}{T_{min}}=\frac{1}{250ns}=4MHz$$

So the data rate is 4Mbits/s.

Note, this is including any protocol overhead bits transmitted, such as framing information, parity bits and idle cycles. So the actual data encoded will have a lower rate.