This is the 35mm box. You would have to buy the 6x6 or 6x7 box (should be available from Fotoimpex) to be able to print 4.5x6.
If you happen to find the 6x9 box (somewhere; it is not available from Fotoimpex anymore), buy it, so you have all possible formats covered. In practice, you gain a little more light (speed) when using the smallest possible size to enlarge (6x6 box for 4.5x6 compared to 6x7 or 6x9 box for 4.5x6), but unless you work with very slow papers (warmtone) or go big enlargements it will not make a big difference.
Other stuff:
I would suggest that you buy the masks for negative mask inserts for 6x6, if you do not already own them still available (AFAIK, Meopta never made inserts for 4.5x6 for the Magnifax, not sure if the Opemus inserts work, probably not). If you have the two glass inserts (standard glass on bottom, Anti-newton on top) you do not strictly need the inserts, but I have found that I have a lot of newton rings with the Meopta glass inserts and I try to enlarge glassless whenever possible. If you decide to use the glass inserts instead, use some thick black paper to mask the area around the negative (this helps to eliminate stray light).
You will need an enlarging lens that covers 4,5x6. A lens for 35mm format will not work. There are some specifically for 4.5x6, but the 80mm lenses that cover the next larger format (6x6) are easier to find and are more versatile, as you can use them to print 6x6 as well in the future. Ask here if you are not sure what you need.
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