Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands	Jan. 31, 2021	Apr. 30, 2020
Securities available-for-sale	$ 23,517	$ 29,204
Investments, Fair Value Disclosure, Total	38,803	31,319
Money Market Funds [Member]
Cash equivalents	15,286	2,115
Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale	23,517	29,204
Investments, Fair Value Disclosure, Total	38,803	31,319
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents	15,286	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