UPDATE: this article is no longer required as 4G for the carrier Proximus and Base are now supported by your iPhone natively!
In Belgium, the carrier Proximus is rolling out the new 4G (a.k.a. LTE) network. Proximus will be using the 1800MHz channels, which is supported by iPhone 5 and a number of Android phones (notably the Galaxy S3 and Note II). Unfortunately, Apple's policy is that carriers providing 4G first have to pass a "quality test" before they are added to the list of "supported 4G networks". Because in Belgium, Apple has a exclusivity contract with Mobistar and the latter aren't rolling out 4G just yet, results in Proximus not being officially supported. As a result, it will probably take some time for them to be added as a "supported 4G network", leaving many iPhone 5 users with Proximus out in the cold.
Currently, some people at userbase have figured out a way to enable LTE (as 4G is called on iPhone) support by using a modified IPCC (carrier settings file). The file itself can be downloaded from their site (direct link).
To install the modified settings file, you need a computer (Mac or Windows computer) with iTunes installed. Also, make sure your iPhone 5 is running at least iOS 6.1.
defaults write com.apple.iTunes carrier-testing -bool TRUEThis will enable the ability to install customized IPCC files on your iPhone.
cd location-of-itunes-exe iTunes.exe /setPrefInt carrier-testing 1This will enable the ability to install customized IPCC files on your iPhone.
When done properly, the Use 3G in Settings → General → Cellular will have been replaced with the Use LTE option.
Note that 4G may not be activated on your subscription. If you have the LTE option, are in a 4G covered area but only have 3G, you can call customer service (dial 6000 on your cell) to have them activate 4G on your subscription.