It all comes down to the fact that Proximus is no "partner carrier" of Apple for its iPhone/iPad devices.
The official list of all Apple's Partners Carriers can be found on Apple's website: http://support.apple.com/kb/HT1937 .
Specific information about a carrier is stored in "Carrier Settings" or "Carrier Bundles".
Based on the MMC/MCN number stored on a (mini)SIM/MicroSim/NanoSIM card, and iPhone knows what carrier bundle to use.
Configuration stored in a carrier bundle are (for example and not limited to) : voicemail number, APN settings, visual voicemail support/information, etc etc.
In the past it was possible to load these carrier bundles on your iPhone/iPad by using IPCC files. Since iOS4 (?) Apple needs to "sign" these IPCC files. An iPhone won't accept the configuration in an unsigned IPCC file.
(unless you jailbreak and patch CommCenter, to remove the check for the signature)
Apple has information about these configurations: http://support.apple.com/kb/ht1970
This is a list of all MMC/MCN numbers: http://en.wikipedia.org/wiki/Mobile_Network_Code
Only Partner Carriers have preloaded their configuration on all iPhone/iPad devices.
These preloaded or "stock" bundles are stored on an iPhone/iPad in this directory: /System/Library/Carrier Bundles/
If a Partner Carrier wants to update this information, it can release a "Carrier Update" and then it will be stored in this directory: /var/mobile/Library/Carrier Bundles/
These Carrier updates can be downloaded from Apple's website: http://iphonediags.apple.com/version.xml and can be delivered to an iPhone/iPad via OTA (in previous versions, Carrier updates were send/installed through itunes.)
An iPhone will check if there is an update for its carrier settings when you access settings > general > about.
(again, jailbreakers can confirm this information)
Carriers that are no "Partner Carrier" of Apple's, fall back to a general set of carrier settings.
This is the reason why we need to manually configure the APN settings of Proximus on an iPhone/iPad.
An iPhone/iPad simply does not have that APN information, because there is no carrier bundle with the information.
Mobistar on the other hand _is_ a Partner Carrier, and that is why the APN settings are configured when inserting a Mobistar SIM in an iPhone/iPad.
For this specific problem (no 4G on the Proximus network on an iPhone/iPad) it comes down to this:
For some reason or another an iPhone needs specific information about the carrier's 4G network to be able to connect. This information is _not_ included in the "general" carrier settings.
I can imagine Proximus has contacted Apple about their settings for their 4G network. But since Proximus is no Partner Carrier, Apple seems not intended to release a Carrier Update just for Proximus.
Remarcable, because they did release a carrier update in the past for the iPad, there is a carrier bundle on Apple's website:
MBP:~ geert$ curl -s -L http://phobos.apple.com/version | grep -i proximus
<string>Proximus_be</string>
<key>Proximus_be</key>
<string>http://appldnld.apple.com/iPhone/CarrierBundles/061-8651.20100903.gftr4/Proximus _be.ipcc</string>
<string>http://appldnld.apple.com/iOS5/CarrierBundles/041-6184.20120608.Ytxir/Proximus_b e_iPad.ipcc</string>
<string>Proximus_be</string>
Anyway, there is nothing we can do. Proximus would very much want to enable 4G on our iPhones, but since they don't do business with Apple, it's likely it will take a while - and it is even unsure it will ever happen.
I already send an e-mail to Tim Cook, and I would like to suggest to anyone suffering from this problem to do the same...