My dd1 was 5 when I had my dd2 so similar age gap to you.
Dd1 got very jealous. She never tried to harm dd2 or anything but the jealousy was definitely there.
I never left mine alone together as a just in case.
When I showered I used to put dd2 in Moses basket on the bathroom floor. Dd1 used to go in and put but I could see exactly what she was doing where the baby was concerned. I only ever had 5 min showers.
Dh worked away Monday to Friday so I had no support in evenings for most of the week and I didn't actually lounge in the bath until dd2 was in a proper night routine and going to bed at certain time etc and dd1 was in bed.
Going to the loo, get a sling for the baby you can literally do anything and everything you would normally do with the baby in a sling. Bit more awkward but doable and the baby is safe then. Or take baby to the bathroom with you and lay them on a mat on the floor.
It's unrealistic to think you will get much time to yourself with a 3 month old baby and a 4 year old.
Also I would take baby yo be checked over in case like a pp said, your 4 year old has Injured baby more seriously - you don't always know with little ones because it's not that obvious. Also could be something else like reflux. It's not normal for a baby to cry constantly