As an indicator of how to dress, the average temperature in Iceland is about as useful as the average number of books published. The thing is, Iceland is like a box of chocolates and you never know what you’re gonna get.
The average temperature in April is 2,9°C (35.6°F). However, Iceland is a wild card and it may get as warm as 15°C or drop down to -20°C. Wind, rain and snow are much more influential on how you should dress and you can get all in one day. So bring something waterproof (pants and a jacket), use layers, wear good shoes and don’t forget your hat and gloves.
Just to clarify one thing. There’s no such thing as bad weather in Iceland, just wrong clothing. This video might be from April.