She seems young for pendulous crop which is due to overstretching of the crop from eating too much. Impacted crop can result from eating long grasses that get tangled in the crop. That can rot and lead to sour crop. Since the crop does not smell bad yet, her crop may just be slow in emptying...
Read Two Crows thread on impacted sour and pendulous crop treatment and prevention, with a BYC search, TheChickenChick’s article by Googling it.
I would isolate her in a crate first thing in the morning. Massage her crop several times a day, give no food tomorrow, just water and a little...
Since she is only 4 months old, I would check her crop early in the morning to make sure that it is mostly empty. What do you feed your chickens, and is the food out all day long for them to take when they want? Some chickens may gorge on treats, or if food is only available at intervals, they...