Thanks so much for your comments and resources. My neighbors confirm that she has basically been howling for 4 hours non-stop.
It's good to know that crating or not crating isn't really the issue, since crating will only stop any eventual destruction but in this situation it would only make things worse for her. I've asked a friend to drop by and check in on her, apparently she was quite anxious but managed to calm down after about 15 minutes, (she stayed for 30). I think that at least reassured her. I've also arranged some play sessions with some of her doggy friends. The link on Puppy Separation Anxiety has been incredibly helpful. I'm looking into setting up a camera to help me see what's going on and doing plenty of training with her, slowly increasing my time away. Thanks again!

My basenji is 5 months, she's been with us since she was 8 weeks old.
She gets plenty of exercise, a few long walks each day, as well as socializing with other dogs at doggy parks a few times a week. She sleeps in her crate every night with the door closed, (goes in and out during the day) and behaves really good at home now, she mostly sleeps or plays with her toys. I stopped leaving her in her crate during my absences during the day because although I was building up progressively, I left her in there once for almost 2 hours when she was around 4 months and when I found her she seemed completely in distress. She was afraid to go back inside, and I even had to leave the crate door open at night a few nights until she got back used to it. Since then I have left her in the apartment (uncrated) for short periods (20-30 minutes max) and she seemed to do very well, a little stress when I left but she hasn't damaged or destroyed anything. I did build up to about an hour or so and everything seemed to be going great.
The problem is I now need to be away for 4 hours a few days a week. This morning I walked her for 45 minutes, then came back and spent plenty of time getting ready so she could settle for her usual morning nap then I left calmly saying "I'll be back". Apparently she howled the entire time I was away and the neighbors are already complaining about the noise.

I'd love to hear your input : is she too young to be left for so long ? / should I re-try crating her even if to me 4 hours seems like a long time to be in a crate / how long do your basenjis howl for when you're away? / will she eventually understand that I'm coming back and be able to calm herself down ? / any other ideas to help her stay calm (I do leave the radio on)

Thanks so much!