Africa
In April, it is spring in North Africa, and the Sahara Desert begins to warm up significantly, at least in the central and southern part. Sunny weather lingers on in West Africa, while the heat and the rains continue to move northwards, from the Gulf of Guinea. On the coast of Kenya, the season of the long rains begins. In Southern Africa it is autumn, and the weather is still nice in areas having a subtropical climate.
Where to go: Algiers, Antananarivo, Asmara, Cape Town, Dakar, Funchal (second half), Johannesburg, Marrakesh, Maseru, Santa Cruz De Tenerife, Tunis
Asia
In April, it's spring in temperate areas, while it's still cold in the large areas having a continental climate. Waiting for the monsoon, it is very hot in India and south-east Asia. Instead, you can go in areas having a temperate climate.
Where to go: Amman, Ashgabat, Beijing, Beirut, Bishkek, Damascus, Dushanbe, Jerusalem, Hanoi, Hong Kong (first half), Kathmandu, Nicosia (second half), Sana'a, Shanghai, Tashkent, Tehran, Thimphu, Tokyo
North and Central America
It's spring in Canada and the United States, but it is still cold in large areas, and the weather is often unstable. In Central America, the temperature increases slightly, and during the month the rains increase south of Nicaragua.
Where to go: Bermuda, Guatemala City, Los Angeles, Mexico City, Miami, San José, San Salvador (first half), Tegucigalpa (first half)
South America
In April, in the tropical zone there are still widespread rains, while it's already autumn in the subtropical zone, so you can go there to find lower temperatures than in summer, although sometimes it can rain.
Where to go: Buenos Aires, Caracas, Lima, Montevideo, Sao Paulo, Santiago
Europe
It'spring in Europe. In April, the thaw generally occurs in Northern and Eastern Europe. In Western and Central Europe, it's still cool and sometimes cold, especially in the first half of the month. The southern countries often experience pleasant weather, especially the Iberian Peninsula, which is more easily protected by the Azores Anticyclone. However, rainy days are still possible everywhere.
Where to go: Athens, Barcelona (second half), Lisbon, Madrid (second half), Milan (second half), Rome (second half), Seville, Granada, Valletta
Oceania
In the Southern Hemisphere the hot season continues at tropical latitudes, with the risk of heavy rains and the passage of cyclones. In South Australia it's autumn, with pleasant temperatures and quite rare rains. In the Northern Hemisphere, for instance in Hawaii, it is spring, temperatures rise a bit and the rains decrease on sheltered slopes.
Where to go: Adelaide, Honolulu, Melbourne, Perth