I have developed my WCF in 4.0 and still it is hosted in IIS 7.
Suddenly the Client requirement changes and now wants to host in IIS 6.
Will I face some hosting problem?
As far as I'm aware you can host a .NET 4 WCF Application in IIS6 but you are limited to HTTP binding.
I'm not sure about this but for .NET 4.0, you might have some problems on running IIS 6 since single application pool can’t use for two different frameworks whereas IIS7 can create Application Pool targeting different framework.
You can do it if its a normal HTTP
binding full detailed explanation is here