I think the best first step is establishing expectations and desires from both sides. Communicating now will definitely help not only with your foundation but also with the future as you both inevitably grow and change. I don’t mean just talking about this a couple of times, but I mean many, many lighthearted conversations with lots of questions (“What if…” ones get to be pretty fun eventually after you’ve established a base understanding of one another). She may not want you involved much, but there are chances she has her own ideas already how you’d be included.
I’d ask your partner about scenarios they imagine as being fulfilling. Ask generally what they wish you’d do for them, what your place or role would be in a situation where they’re feeling regressive. What do they like their Caregiver to do for them and what do they like to do for their Caregiver. A good question might simply be what they generally like to do when feeling regressed so that you can imagine how you’d be positioned in that moment and then discuss if that lines up with her thoughts. After all, nobody else truly knows what’s going on in her mind and what exactly it is she’s been dreaming of when it comes to having a partner interact with her during her regression. Have fun with it, make conversation and bond over those things that play out in your minds!
The most common things I see are watching cartoons/animation together, a Caregiver preparing snacks or meals, a Little coloring/drawing pictures for their Caregiver to be “proud” of, a Caregiver brushing the Little’s hair, the Caregiver reading bedtime stories, and potentially the Caregiver setting
helpful rules that are intended to give the Little better structure or guidance (and, pssst, if a rule is broken it doesn’t mean she has to be punished if neither of you are into it). Not all Littles like these more common things though so you’ll have to ask her and try things out if she’s not sure. You may also consider some general things that show an individual cares about another individual can play into her feeling valued, even if it’s asking how she is, how her day went, or if she’s looking forward to anything coming up.
If she eventually wants you to help her “explore” feeling more regressed to 4 years old then you might ask Google something like “activities for 4 year olds”. I always find a good handful of doable activities, shows, or even conversations to have just by searching as if my partner really is X years old and I’m trying to entertain them.
Not all regressive feelings need to be met with attempts at immersion. What I mean is that if she’s feeling a little bit little it doesn’t necessarily mean you have to be involved or have to encourage her to feeling it more. These things definitely fluctuate and are not always very immersive or even obvious. Sometimes your expected role is just to simply be yourself and accept that she’s expressing herself a bit differently than that expected of an adult.
Just like acting out as a child isn’t a game to her, being a Caregiver isn’t a game to play either. It doesn’t have to be physically intimate, doesn’t have to involve domination or submission, and isn’t always tied to kink.
Caregiving is definitely an act of expressing love so don’t feel pressured to push things further than you want at a time. Some Littles may get very excited initially and ask for many things all at once and that can put you at risk of
feeling burnt out, or underappreciated or unloved in return. Just the same, some Littles may feel shy, timid, or want to keep their regressive feelings more to themselves. It’s okay to give love in other ways, and you should definitely expect fair treatment and a fair amount of reciprocal love in return however she expresses her feelings for you too. Enjoy getting to know this part of who she is and let that goal be your guide for now.