>I see that in my Microsoft.net framework I have .net version 4.0.??? Many of the settings in my ASP.NET project have targetFramework="net45"
>Does it mean that I would have fewer issues if I upgrade .NET to 4.5?

I didn't think you could target 4.5 if it wasn't installed. IAC, go for 4.5 - you may not get fewer issues but you won't get more :-}

For me the biggest improvements in 4.5 were added async / parallel capabilities but there's also the new System.Net.Http namespace.