A 4 year old should be able to shower themselves or bathe themselves but they usually also need some help in the shower or bath.
Also never leave the 4 year old in the shower or bath by themselves as they could potentially drown in the water.
My now 6 year old boy would shower by himself when he was 4 years old but sometimes needed some help washing his hair and making sure he soaped up.
I would sometimes step out of the bathroom for a bit but would always leave the door opened.
I mostly stayed in the bathroom with him and helped him as he showered or bathed.
I even showered by myself when I was 4 and 5 years old although sometimes my mom would have to make me take a bath or shower and give me the bath or shower herself.
I was capable of bathing or showering myself but sometimes I would just refuse to take the bath or shower.
Some kids will also fake like they are showering or bathing when in fact they didn't actually take the bath or shower so you sometimes have to be in the bathroom with them to ensure they actually get clean.