Well, April's a good time of year across the globe for "not too hot, not too cold" weather. Even in the southern hemisphere, so I vote for
Australia. April's avg. temps are a high of ~75 and a low of ~55-57. The only downside is it averages nearly five inches of rain in
Sydney. But if you go to the other side of the island (say,
Perth), you'll get less rain and moderately similar temperatures (maybe a little warmer, but not much).
Japan, more specifically
Tokyo, is about 10 degrees cooler on average and also averages a bit more rain. I had no idea it rained that much in Tokyo until I checked. Makes
Seattle look like a desert.
If you want to go in the other direction, like Europe, you'll get temps in the 60s and not much rain. Greece, for example, is nice this time of year and you'll beat a lot of the tourists.