Ah..fruit trees..my favourites... ;D
Ok, first..do you know what rootstock your trees are crafted into? Gisela5 is the smallest for the cherries and Pixy for the plums....BUT...even it is said they 'only' grow certain size..I would read the meaning as 'can be achieved to keep within'. Growing conditions and crafted variety will have some saying too how much the trees grow.
Plums in Pixy rootstock are expected to grow 8-10 ft...Gisela 5 about 6-8 ft...the spread of the canopy is about same as the height and ideally you want to have bit of gap between trees too after they've grown into full size trees.
So if you start calculating by the maximum spread..remembering they often do grow larger than is the guide.
4.5 m = about 15-16ft..so if you plant plum in two opposite corners of you area..that will leave just about enough room between them, but the canopy and roots will venture outside that 4.5m square area as well.
Although the cherries should stay bit smaller..I would plant them same..to the opposite ends of the area.
So each tree in each 'corner' of the plot and eventually, even if they look bit lost in space, they will fill the spot and bit more. So sadly no more fruit trees on that area and you need to do some amount pruning to get them into shape in their early years. If you are not confident about pruning..we can get to it once your trees have put some new growth on..propably no need this year at all.
BUT..you could squeeze something else 'out' of that space..planting perharps a gooseberry bush in the middle OR strawberries OR couple blueberries in containers perharps some herbs like chives or parsley.
You can crop the middle area with veg..until the trees start to get much bigger and the roots gets damage from the digging..but that is some years ahead... ;)
