If you cracked your Apple iPhone 4's screen (actually it is the digitizer that comes with an LCD), we at Dr Mobiles Limited can replace it in less than one (1) hour; actually it is more like 45 minutes or so. On top of that we use only ORIGINAL parts (same as those used by Foxconn Technology).
Besides that, you can avoid taking risk using dubious or semi-pro who might damage your expensive iPhone 4. If those "pro" repair centre cannot do the replacement on the spot in less than an hour; it is very very likely they have to send away your expensive iPhone 4 to another location for repair (means increase risk of lost and pay $90 to $150 more)!
Please remember, cheap does not mean good; greed will ultimately make you suffer or have your expensive iPhone 4 permanently damage by shoddy repair centre! Use your wisdom and think carefully before sending your expensive iPhone 4 for repair, it could be a one-way trip for your iPhone 4!