Note 11 - Treasury Stock and Repurchase Program - Treasury Stock at Cost (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			9 Months Ended
	Jan. 31, 2021	Oct. 31, 2020	Jul. 31, 2020	Jan. 31, 2021	Jan. 31, 2020	Apr. 30, 2020
Balance, shares (in shares)			383,279	383,279
Balance, total average cost assigned			$ 5,957	$ 5,957
Balance, average cost per share (in dollars per share)	$ 16.66			$ 16.66		$ 15.54
Balance, aggregate purchase price remaining under the program	$ 955	$ 1,578	$ 2,000	$ 955		$ 2,000
Purchases effected in open market, shares (in shares)	21,411	15,670
Purchases effected in open market, total average cost assigned	$ 623	$ 422		1,045	$ 572
Purchases effected in open market, average cost per share (in dollars per share)	$ 29.10	$ 26.96
Balance, aggregate purchase price remaining under the program	$ 955	$ 1,578	$ 2,000	$ 955		$ 2,000
Balance, shares (in shares)	420,360			420,360
Balance, total average cost assigned	$ 7,002			$ 7,002