Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands	Apr. 30, 2019	Apr. 30, 2018
Securities available-for-sale	$ 21,828	$ 17,844
	27,445	22,826
Money Market Funds [Member]
Cash equivalents	5,617	4,982
Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale	21,828	17,844
	27,445	22,826
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents	5,617	4,982
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