@Smurf123 Joie 360 inserts - these are a bit trickier than most seats so don't worry! I'd put both the body and wedge insert in for a 4mo just as a guess without seeing. I will link to a video in a bit but this is what I tend to go by:
Head insert - use to support tiny wobbly newborn head, remove somewhere between 3-6 months. Note that if you need to move the headrest up you can move the head insert up, as it has 2 poppers on it which set it at different heights. Take out when it's squishing the head or getting in the way.
Body insert - second one to go, you keep this in to fill up space at the sides between the child's body and the side of the seat. This is actually hugely variable as if they are really skinny you might use it until they are about 2. Most children need it in until a minimum of 8-9 months. If their arms are awkwardly pushed up by it try without.
Wedge - really trips a lot of people up, but intends to replicate the extra recline from an infant carrier, so you really want it in until around a year old. Minimum requirements to remove:
Baby at least 9kg
At least 6 months old
Sitting unaided
No longer using the body insert
If you take it out and his head flops forwards while sleeping, put it back in.
Don't forget also you can move the headrest up. The pads on the straps can be tricky as they are bulky. Try to aim for a position where the straps are level with the child's shoulders. If the pads are all bunched up in the inserts, I find it helps to take them off and move them very slightly down so you have a tiny bit of bare strap at the top to work with. But they should start on top of the shoulders and curve down. If they are going up and over and down the back, move the headrest up.
Sometimes they are kind of sitting on the buckle and that causes people to want to take the wedge out. It's annoying, but don't. It's a bit of a flaw in the design. Try putting the headrest up one notch and placing him higher in the seat.
Also obviously you wouldn't put him forward facing yet, but if/when you do move to forward facing, no inserts are to be used, so you need to wait until they are no longer needed :)
I used ours without inserts for DS2 who was 8 months when he went in it, but looking back he was too small and needed both the wedge and insert.
(Photo 1: 8.5 months, needed both insert + wedge, headrest slightly too high. Photo 2: Around 20 months, looks right here without either.)
And the lovely Margaret.