There are some things you can do to strengthen the association in the dog's mind between the command and obeying the command. First off you're training her in the garden with no distractions which is good, but at this intermediate stage, when she's in the park or at the beach you should only call her back when you're sure she'll return, like when she's already running towards you or when she's paused to decide what she'll do next. DO NOT CALL HER if you think she'll have trouble returning, like when she's found something to sniff or another dog to play with. All this will teach her is that it's ok to disobey you. Rather than expecting her to return every time, don't give her any opportunity to disobey, issue the command so that it's easy for her to obey. When she follows you when you're moving on for example, call her and make her think she was obeying the command to come.
Don't keep calling her over and over again when it's obvious she doesn't have any intention of returning, this turns the command into annoying background noise for the dog, and she won't learn to associate it with obedience. You should only be issuing the command 3 times at maximum, after this you will need to approach her in a totally neutral non-angry way (without speaking or making eye contact) and put the lead on her for about three minutes. This will teach her that a consequence of disobedience is loss of freedom. Walking on the lead for a few minutes will also calm her down and reset her state of mind to one that's more open to commands.
At this stage ALWAYS make a huge deal of it with pats and praise and a smelly treat such as dried liver when she returns on command, even if it wasn't immediate. The idea is that returning to you should be better than anything she could find on her own. Act like she's just turned coal into gold, and she'll associate obedience with a very positive outcome. With a lot of repetition she will start returning as a reflex rather than weighing up whether its worth it or not and only returning if she feels like it. You will notice that the percentage rate of her obedience will increase. At 4 months this is an excellent time to start teaching her this command and within a year, with enough persistence, she should be almost 100% reliable. Start them off young I say.