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, 2019	Oct. 31, 2018		Jul. 31, 2018		Jan. 31, 2019	Jan. 31, 2018	Apr. 30, 2018	[1]
Balance, shares (in shares)	[1]				308,380		308,380
Balance, total average cost assigned	[1]				$ 4,135		$ 4,135
Balance, average cost per share (in dollars per share)	[1]				$ 13.41		$ 13.41
Balance, aggregate purchase price remaining under the program		$ 1,715	$ 1,715	[2],[3]	$ 209	[2]	$ 1,715		$ 255
Purchases effected in open market (in shares)		13,438			2,286	[2]
Purchases effected in open market, average cost assigned		$ 285			$ 46	[2]	$ 331	$ 191
Purchases effected in open market, average cost per share (in dollars per share)		$ 21.20			$ 20.26	[2]
Balance, shares (in shares)	[2],[3]	324,104					324,104
Balance, total average cost assigned	[2],[3]	$ 4,466					$ 4,466
Balance, average cost per share (in dollars per share)	[2],[3]	$ 13.78					$ 13.78

[1] Includes 85,219 shares with a total average cost of $1,036,000 that were acquired during the former repurchase program which was authorized in January 2011 and expired in January 2012; 18,400 shares were acquired prior to January 2011.

[2] Includes 207,047 shares with a total average cost of $2,792,000 that were acquired during the $3 million repurchase program which was authorized in September 2012 and expired in October 2018.

[3] On October 19, 2018, the Company's Board of Directors authorized the repurchases up to $2,000,000 of shares of the Company's common stock of which 13,438 shares with a total average cost of $285,000 were purchased through January 31, 2019.