My Scottie (boy) has been trained to potty only outdoors via crate training. When I got Casie (girl), she was more difficult to crate train for outdoor only, but she had already knew how to use the pad. So now, she actually goes on both the pad and outdoors. She'll pee on the pad when I'm away from home. But she'll potty when I take her outside at the same time every day. From my experience, it's possible for Casie to do both.
To prevent accidents, your baby needs to learn that the pad is always in the same spot and that's where she should go potty. So, if she's heading to the pad, I'd let her. If you catch her using the pad, always celebrate and tell her "good girl!!!". It's also important that her potty pad is close enough in the area she's in so that she can reach it in time when she needs to go. That's why it's best to restrict their access to the rest of your home. Otherwise, to them, it might be like a child being in a huge mansion with ONE toilet and you don't know where it is.
Then, take her out for scheduled daily walks (i.e. I do 630am, 4pm, 8pm, and 10pm walks because Scottie only potties outside). If she potties outdoors, always do the same celebratory "good girl!!!!". Eventually, she'll learn that pottying indoors on the pad and outside are both 'good'. Granted, my neighborhood has A LOT of dogs (i.e. lot of dog 'smells'). So it wasn't difficult for my babies to catch on that the great outdoors is nature's dog toilet.
If your baby is still 4 months, have some patience. It'll take some time, maybe a few months for her to 'get it' and for her to develop the ability to 'hold it'. Always keep her tethered to you an hour or two before her next scheduled potty break (i.e. if it's almost time for their walk, I keep them on the couch with me, because I know they won't soil the couch). If you or your kids ever catch her in the act of an accident, do a very firm "NO!!", pick her up, and quickly take her to the potty pad.
Oh and feeding at a regular schedule helps to regulate their potty schedule too.
Just be aware that even at the 1 year old mark, she may still have the occasional accident after she 'gets' it. Just make sure you always clean up all accidents with products like Nature's Miracle.
Good luck!!