How can I proportionally reduce a picture's size (e.g.--go from 5x7 to 4x6)

In iPhoto

Select photo > Edit (button on bottom) > Crop > Constraint > pick size and drag window where you want

In Preview

Edit button >Tools > Adjust Size > Type size

You are going to experience some cropping when going from 5x7 to 4x6 as in your example because the proportions aren't matching (5/7 <> 4/6). I don't mean to be "that guy", but if maintaining the entire composition of the photo is what you're trying to do, then simply changing dimensions is going to result in some loss.

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Of course, as mentioned 5*7 & 4*6 aren't proprtional, if you scale 7 to 6 you get 4.3*6, if you scale 5 to 4 you get 4*5.6, of course you can still make it 4*6, but not proportional...

As Creeper74 said those two ratios do not match so you will distort the image slightly by doing this. Also as others said there will not be a settign to specifically do this, however with a little maths you can do it.

First find the number of pixels high and wide your image currently is. If it is currently 5x7 this should mean the height is 7 5ths the number of pixels it is wide (or vice versa depending on whether we are talking landscape or portrait orientation). So using hypothetical figures if it is 100 pixels wide then 100 divide by 5 x 7 = 140 high for the current 5x7 ratio.

To convert it to 4x6 take the 100 wide divide by 4 and then multiply by 6 which gives you 150 high. Therefore you need to resize from 100x140 to 100x150.

You would have to turn off constain ratios to do this.