I've had a 4in one and a 5in one but never a 4.5in one - would it be a rarity or rather scarcity?
There is another way to determine what size it is. You firstly have to remember that the sizes given are
purely arbitary and were never used by Chance anyway - thay called them 'Small' (4-inch, 10cm) and 'Medium' (7-inch, 18cm)) so I'm not sure where the 5-inch is supposed to go... medium-small, perhaps?
If you measure the base from the inside, it will be a tad over 2" (5cm) for a 4-inch posy bowl. The inner base of the 5-inch should be about 2½" (6cm).
I imagine yours is a slightly 'peaky' 4-incher!