I'm not sure what it'll be like in April but one of the less-publicized hikes that I think is quite lovely (and, I would expect it to be at least largely snow-free in April) is to the top of Chilnualna Falls down in the Wawona area. There's a lot I love about this hike:
* Even though you gain about 2700', you do it mostly quite gradually so, even if you're unaccustomed to higher altitudes, this one won't get you huffing and puffing like Darth Vader.
* You'll pass through almost every type of scenery available in Yosemite (except Giant Sequoias but when you get back down, you're only about 6 or 7 miles from the Mariposa Grove).
* There are stunning views of the Wawona Basin almost the whole way up.
* You have a lot of options for how far you can continue going once you're at the top.
Depending on your hiking speed, this can easily be a 6-8 hour hike if you go beyond the top of the falls and dally around up in that area.
If you DO go to the Mariposa Grove (and I strongly recommend you do), be aware that there's an upper and a lower grove. The famous Grizzly Giant and the California Tunnel Tree are both in the lower grove and most tourists huff and puff their way to them (they're right next to each other) and then turn around. The Giant IS a magnificent tree even if it seems like it is entering its final years (or, more likely, final centuries) but these days I usually head straight for the upper grove where, except for the busiest times of year, you'll see a lot fewer people. For an added dose of privacy, bypass the lower grove via the old stage coach which leaves from the upper left portion of the parking lot (i.e., if you have your back to the entrance road and straight ahead is 12 oclock, the old stage road is at about 10 or 11 o'clock). The road itself is a beautiful step back in time and I've never seen more than a handful of people on it. I believe it's called the outer loop on the trail maps but I wouldn't swear to that off the top of my head.
The Four Mile trail is one of my favorite hikes out of the Valley (although, like most of the south-rim hikes, it'll probably have snow and ice in April) but I also recommend the hike to Yosemite Falls. And don't blow-off the Valley Loop trail just because it's flat (I'm not saying you WOULD but I skipped it for several years because it was "too easy"
)...you'll see stuff on this route that you'll never notice from your car.
Everybody's got their favorite hikes in the Park and, I suspect, most of us who have been there many times probably base our "favoriteness" on how few people you'll see but, in general, April won't be too crowded and, hell, there's a REASON why so many people flock to the Valley every year...every square foot of it is STUNNING! Don't be afraid to explore the places that everybody else goes to (I even did the tram-ride around the Valley tour the first time I was there and came home with such beautiful pictures that I couldn't wait to get back. I've now been going back regularly for several years (even though I live in NJ)). There are more and less demanding hikes in Yosemite, there are some that are best at certain times of year but there are no bad hikes and I can't imagine you'll feel like you wasted your time no matter which ones you do.
Please take lots of pictures and share them here!
--David