Hardik,
Please clarify your expected data throughput. You mentioned: "need at least 4 channels on the ADC where I will input 20-30 Khz four analog signals ... simultaneous sampling and the sampling rate should be at least 250 kSPS for each channel". How many bits per sample? I assumed the combined data throughput would be higher than 4 * 600Kbps per channel or 2.4Mbps. If it is this low, then SPI is an excellent choice.
Tom