Hi, it would be quite unusual for her to be in season at only 4 months old. Dogs hump each other for various reasons so I wouldn't use this as a sign. The physical signs of a season are some blood and swollen lady bits. There are some behavioural changes too to look out for but I would be surprised if she is, worth having a look though.
Girl dogs will hump...also for other reasons than it being her season.
Lady will still occasionally hump on of her large teddy's.
but I am not sure about the leg thing...lol other than for a belly rub.
She is young at 4 months to have her first season.
4 months would be extremely rare for a first season.Early signs my girls show are swollen girlie bits,scent marking on walks( doing lots of random small wees) they all start humping each other( and theyre all girls!!!) and their moods change too,they can get a bit grumpy xxx