Share your experience!
OK, we'll be waiting! 
this is intriguing, I would say it has 4GB for the user to stored anything and the other 4GB are divided between OS and perhaps app space.
http://www.sonymobile.com/gb/products/phones/xperia-u/specifications/
So, what you're trying to say is the 2 GB system memory is for the OS and the 2 GB phone memory is for apps?
exactly
Are you sure about this? Because if you are, I would choose your reply as the correct answer. 
that'll be great but you can wait for confirmation from Johan, but if you check the link
http://www.sonymobile.com/gb/products/phones/xperia-u/specifications/
it will add up 4GB for music and pictures and the other 4GB subdivided 2GB for Android and 2GB for apps
System memory 2 GB
Phone memory 2 GB
Internal storage 4 GB
Apparently it's taking him a while to confirm. 
I read this from xperiablog.net regarding Xperia U's memory:
"The Sony Xperia U only has 8GB of in-built flash storage and no micro-SD memory card slot. Only 6GB of this is user accessible with 2GB of this allocated to phone memory for apps, data and games and the balance for media."
http://www.xperiablog.net/2012/03/06/xperia-u-our-hands-on-impressions/
According to this, U has 6 GB of user accessible memory, where 4 GB is the internal storage where user data is stored and the 2 GB is the phone memory where only apps and apps data can be stored. I do not know if this is true but it sounds probable.
I was right
I cannot mark your reply as the correct answer yet, though. We need the confirmation. 