I recommend Amelanchier lanarkii . . . has pretty white flowers in April, and autumn colouring. It can be grown as a tree or shrub (there are a couple in my photos).

I'd agree with Sheila's recommendation above. But, and its a big but, your bloom time is very specific - end of April - and it can't be guaranteed that shrubs will definitely be in flower at the end of April - in a mild year, in a very sheltered town garden, Amelanchier might even be finished by mid April - in a more exposed and colder region, it might only begin at the end of April. This is true of many other shrubs which bloom around that time, so the situation you want to plant in is a factor because of micro climate. For instance, Berberis darwinii can start flowering the last week in April - other years it's mid May, again depending on micro climate and whether its a cold or warm spring.

Have a look at the smaller flowering cherries, because some of them are later than others, and it'll be a late flowering one you'll be after, and the crab apples, most of which are April into May flowering. All crab apples are very pretty in flower, so it'll be the fruit which will steer your decision. Look at Malus 'Red Sentinel', and M 'Profusion'. Worthy