I tried waitforcompletion on wfa 4.0 as you mentioned and it worked for me.
Now to debug this, can you tell me what is your client being used to call this API? Is it a powershell code? which version of powershell do you use. Can you share you code snippet. You can PM me if want.
The thing is WFA4.0 has upgraded some of its building block infrastructure sw. And we had encountered an error previously. So , let see
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I see this as an issue with your Client ( python suds module ) with WFA4.0 Jboss 8.2.0. Not 100% certain but in the past I had come acoss an issue with Powershell and Jboss7.1 and that was showing similar behaviour in terms of not working with the latest WFA but working fine with the older one ( having older Jboss version)
I've tried the said APIS with Powershell and it works all fine. Also tried with other SOAP UI clients and they also work fine.