I-Size relates to the construction/size of the child seat and improved protection in the head area, plus child measurement is done on height rather than weight, with rear facing for a minimum of 15 months.
A part of the i-Size regulation means that car manufacturers should provide seats(in the car) that can accommodate these newer style of seats (for the child), but unlike ISOFIX it doesn't require a specific set of hardware, more that there should be enough space in the car to accommodate it. So, in a compact car the rear seat/bench is unlikely to be i-Size compatible (even if it has ISOFIX) as there won't be enough. room between the back of the driver's seat and the back rear seat backrest to accommodate an i-Size car seat, especially if the driver is over average height.
(Some of this you may already know of course)
That all being said you can still use an I-Size seat on a car that doesn't have i-Size compatibility confirmed, as long as there is enough space. And while I have no idea of the rear space in an ID4 I can tell you that a Nuna Rebl, which is an I-Size seat, fits perfectly well in the back of a Mercedes B250e and a Mk5 Golf with space to spare so I am sure you'll be fine.