My son had a little wooden heart (the sort often sold in church shops) at this age for this reason. I think we may have had a bit of a ritual of me kissing it the night before when we were packing his bag but I actually can’t remember clearly now. It helped. There was definitely an element of performance/habit to it with him, and his nursery nurse was always great at peeling him off me at the door with a cheerful smile. It passed within weeks but was horrible at the time.
We used the children’s meditation book ‘in my heart’ at that time (coincidentally, more because he had always found it quite hard to unwind and get to sleep). One of the chapters in there is about connectedness when we are apart from people we love, so I read that one a lot and used the imagery to talk about missing mummy while at school.
I also tried different ways of demonstrating that he was in my mind whilst he was at school - things like little notes in his lunchbox, showing a photo of something I’d seen during the day which had made me think of him (often this was quite tenuous
), occasionally turning up to collect him with a random small treat (I don’t really want to overuse consumables/food as consolation/create an expectation around that, but again it was along the lines of ‘I went out on my lunch break and saw this and thought of you...’).
At 7 mine still sometimes gets a bit stroppy in the morning (at home) “I don’t want to go to school...” and I tend to agree that I’d much prefer it if we could just run away to the seaside or whatever, but [bright and breezy] them’s the breaks, I have to go to work and he has to go to school. But I’m not sure how well that would’ve worked at 4.
I think there’s a bit of a question around the balance between validating their feelings vs not enabling them to become too entrenched in an “I’m so sad about school” mindset. Hard to judge without seeing it first hand (IME hard to judge even if when it’s your own child!), but throwing it out there as a thing to consider. Sometimes mine needed chivvying out of it but other times that would be inappropriately dismissive.