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Note 17 - Concentration (Details Textual) - USD ($)
12 Months Ended
Apr. 30, 2019
Apr. 30, 2018
Apr. 30, 2017
Revenue from Contract with Customer, Including Assessed Tax $ 36,257,000 $ 35,868,000 $ 34,574,000
Customer Concentration Risk [Member] | Revenue Benchmark [Member] | One Single Customer [Member]      
Concentration Risk, Percentage 20.50% 17.70% 10.30%
Revenue from Contract with Customer, Including Assessed Tax $ 36,257,000 $ 35,868,000