What is he going to achieve with a 4 ft pipeline?
Imagine a theoretical flow of 1 meter per second - a guesstimate, I'm not qualified to calculate the amount of pressure the pipe would have to withstand for that kind of flow. In that case the pipe would deliver about pi * (4 * 0.304) ^ 2 = 4.6 m3 per second, or about 1200 gallons per second. I looked up the water use of various crops in California: http://www.nass.usda.gov/Statistics_by_State/Arizona/Publications/Bulletin/11bul/pdfs/56-field%20crops-narr.pdf
Wheat uses 435,000 gallons per acre per season. Assuming a season is about 5 months, or about 13 million seconds, the pipe would deliver about 16 billion gallons, enough to irrigate 37000 acres, assuming 0% loss. According to this: https://4aa2dc132bb150caf1aa-7bb737f4349b47aa42dce777a72d5264.ssl.cf5.rackcdn.com/aft-ca-agricultural-land-loss-basic-facts_11-23-09.pdf that would be less than one tenth of a percent of agricultural land in California.
I looked up the yield of various crops: http://www.nass.usda.gov/Statistics_by_State/Arizona/Publications/Bulletin/11bul/pdfs/56-field%20crops-narr.pdf I couldn't get a clear figure for California, but the average seems to be $8.35 per acre (assuming per season). For the 37000 acres irrigated by this pipeline this translates to about $310,000 per season. Assuming zero maintenance cost, the pipeline would start paying for itself after about 97000 seasons.
Even if my assumptions are off by a factor 10, and if I made a calculation error of another factor 10, this would still be a very poor way to invest $30 bn.
There's probably a good reason why irrigation is more often done with 20ft canals rather than 4 ft pipelines.