If money is no object, get an Imacon or even better, a drum scanner!
But if you are like the rest of us, especially for 4x5's, you can easily get away relatively cheap flatbed scanners.
Personally, I use a Canon 9950f, which does a great job and is really cheap for a film scanner. 4x5 film is much easier to scan than smaller formats like 35mm, so you can easily get away with a flatbed scanner for it. Plus, you can also use it for scanning prints.
If you want the best you can get out of 35mm, you should invest more - for best results, a lot more. But for web publishing, the Canon does just fine even with 35mm.
Check my gallery - all the images posted there are scans of various film formats with the Canon 9950f, then leveled, spotted, and sharpened in Photoshop. I have a mix of 35mm, 6x6, and 4x5 scans.
-chuck