Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands	Oct. 31, 2020	Apr. 30, 2020
Securities available-for-sale	$ 29,485	$ 29,204
Investments, Fair Value Disclosure, Total	37,468	31,319
Money Market Funds [Member]
Cash equivalents	7,983	2,115
Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale	29,485	29,204
Investments, Fair Value Disclosure, Total	37,468	31,319
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents	7,983	2,115
Fair Value, Inputs, Level 2 [Member]
Securities available-for-sale
Investments, Fair Value Disclosure, Total
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash equivalents
Fair Value, Inputs, Level 3 [Member]
Securities available-for-sale
Investments, Fair Value Disclosure, Total
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash equivalents