Bear with me for a second here for some background....
When you downsample a 4000×3000 image to 400×300, you are "discarding" 11.9 million of the 12 million pixels. This clearly reduces "image quality", depending on what exactly you mean by that term.
If you go from 1000×750 to 400×300, you're reducing the area by about 6 times. Again, data is discarded, but not as much.
Now, if your original image had a lot of defects — noise, aberrations, and focus error — the resolution may exceed the "effective image quality", and the extra detail is contributing nothing. In that case, downsampling to some degree doesn't "really" reduce image quality. This is very dependent on the actual original, though — and in any case, going all the way down to a basically-thumbnail-sized 400×300 is pretty drastic.
Hopefully that's all pretty obvious. With that out of the way... In this case, since 400×300 images are your target, the question is really "is there any value in having larger images and downsizing them myself rather than having the stock photo agency do it for me?"
The answer to that is: maybe.
On the side of buying large and scaling down:
It's possible that the agency uses poor downsampling and sharpening algorithms for the smaller choices. Or they may use perfectly good algorithms that have the wrong parameters for your taste — for example, too much sharpening. By picking a bigger version and downsampling yourself in Adobe Photoshop or dedicated scaling software, you might be able to do a better job, and tailor the results for your artistic needs and desires.
Starting from bigger pixel dimensions allows you more flexibility, since you can crop and still have room left over. You could decide that just the corner or the center of an image fits your use better, and crop out a 400×300 section rather than downscaling.
Likewise, if you're doing other manipulation on the image, to change color or make a montage from multiple images, having some extra data to work with at that stage won't hurt (although beware of spending too much time getting detail right when it's just going to get scaled out of perception in the end).
On the side of buying closer to the final size:
Price — the microstock agency you're using has a complicated pricing model, but the part where lower resolution costs less is pretty straightforward. No need to spend for pixels that aren't getting used.
Less work — if you can buy exactly the size you need, you can just drop the image into place. For many situations, this won't be the case (since the range of sizes offered is limited), but it might be for you.
Ultimately, the answer will depend on your particular situation and how it relates to these factors, but hopefully this will give you a way to make that decision.
