I also think 1800 x 1200 is enough for 4R print. I'll always resize my images to that ratio all the time before bringing them to photo printing. Sometimes I will go for 10R using the same ratio and no prob so far.
depends on what paper size are you printing it, there are two things to consider, #1 image ratio and paper size ratio, #2 resolution.
#1, you are using a canon 450D, the image ratio is 2:3, so if you print a 4R (4"x6") photos, S8R (8"x12"), since the paper size having the same ratio, you need not re crop your image, but if you want to print other paper size like 5R (5"x7"), or 8R (8"x10"), you need to crop it yourself before you send it for print, altho the lab can crop it for you, but it may not in the way you want it.
#2, the minimum requirement for decent quality printing is 250ppi, usually we give 300ppi for easy calculation. so a 4R (4"x6") photo, you need to have 1200x1800pixel, a S8R (8"x12") you need 2400x3600pixel, you can give higher than 300ppi, for an example, you can send a original large jpg to print a 4R, but you can't give lower than 250ppi. the photo it will be pixelated.