Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands	Jan. 31, 2020	Apr. 30, 2019
Securities available-for-sale	$ 25,845	$ 21,828
	28,584	27,445
Money Market Funds [Member]
Cash equivalents	2,739	5,617
Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale	25,845	21,828
	28,584	27,445
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents	2,739	5,617
Fair Value, Inputs, Level 2 [Member]
Securities available-for-sale
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash equivalents
Fair Value, Inputs, Level 3 [Member]
Securities available-for-sale
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash equivalents