The upper slopes of the volcanoes tend to be a lot colder, with food more scarce at this time, so the gorillas stay lower, for food and warmth. This does mean that treks at this time can be a little shorter if you want to risk the heavy rains. However, shorter treks are not guaranteed, this is wildlife and it does as it pleases! You can also find that the gorillas may want to avoid the rain too, seeking shelter and hiding under the canopy.
Similar to March, this is still a good time to see chimpanzees They tend to stick together in large groups and stay lower to the ground at this time, making spotting them a little easier in the wild forests. The rest of Rwanda can be good to travel too. If you are not planning on seeing the gorillas during your time in Rwanda, April is fine to travel.