Note 11 - Treasury Stock and Repurchase Program - Treasury Stock at Cost (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			9 Months Ended
		Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Jan. 31, 2020	Jan. 31, 2019	Apr. 30, 2019	[1],[2],[3]
Balance, shares (in shares)	[1],[2],[3]				336,439
Balance, total average cost assigned	[1],[2],[3]				$ 4,743
Balance, average cost per share (in dollars per share)	[1],[2],[3]				$ 14.10
Balance, aggregate purchase price remaining under the program		$ 866	$ 1,045	$ 1,346	$ 866		$ 1,438
Purchases effected in open market, shares (in shares)		7,523	12,636	3,857
Purchases effected in open market, total average cost assigned		$ 179	$ 301	$ 92	$ 572	$ 331
Purchases effected in open market, average cost per share (in dollars per share)		$ 23.85	$ 23.78	$ 23.88
Balance, shares (in shares)		360,455			360,455
Balance, total average cost assigned		$ 5,315			$ 5,315
Balance, average cost per share (in dollars per share)		$ 14.75			$ 14.75

[1] Includes 207,047 shares that were acquired during the $3 million repurchase program which was authorized in September 2012 and expired in October 2018.

[2] Includes 49,789 shares that were acquired during the $2 million repurchase program which was authorized in October 2018.

[3] Includes 85,219 shares 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.