A professional iPhone repair service can be found in most major cities and in many of the smaller ones also. You can do an online search to find an iPhone repair that is near to you. You do have to take that iPhone into the shop. Or as an option you could mail it to them but it makes more sense to actually take it in your self. You may have to do without it for at least 24 hours. You just have to choose a repair shop that does repairs to iPhones.
Some places specialize in them whereas others can repair most models of cell phones including the iPhone. If you have an iPhone 4 you may have to find one who has a lot of experience on this generation iPhone. When you take it in they will ask what is wrong with it and if you know then tell them. But they will still hook it up in their shop to check it out for all of its functions.
There are lots of things that can go wrong with an iPhone 4 that you may find you need to look for an iPhone repair store nearby. You may have a LCD screen that begins to bleed colors into each other or one that the liquid crystals begin to get green or another color for a weird screen effect. Generally you can still use your iPhone until you can get that screen replaced unless you have dropped your iPhone and the screen cracked or broke but if your cell phone hit the ground hard enough to break the screen then you may have to get your entire iPhone repaired or replaced.
There are other things that can break on the iPhone 4. For example the battery may go out on it. It has a rechargeable battery but that does not mean that the battery won't always recharge. It may wear out or have other problems with it. The battery is not meant to be replaced by the owner but it is designed to be taken into a repair shop or in the manufacturers opinion taken back to the Apple iPhone store for a replacement battery or if at the Apple iPhone store they may talk you into getting a new iPhone currently probably the iPhone 4.
An iPhone repair store won't try to sell you a new phone unless you really need one and your cell phone is not repairable. But these repair store techs are really amazing and can repair almost every problem.
Some of the other problems that may affect an iPhone are that since they are heavily used for many applications they are subject to being dropped, run over by a car, or even eaten by the dog. Some of these can be repaired but some like if the dog has thoroughly chewed up the case and got his fangs into the circuit board then you may not be able to do anything but get a new phone.