Which Library/.NET Versions

Hi,

I am new to Xamarin.
I want to create an app for API Level 21 using Syncfusion Xamarin Components.
When I create the new Project in Visual Studio, which target Framework should I use?
As Syncfusion needs at least Xamarin.Forms 1.4.* i installed nuget package 1.4.4.6.449
Then I tried to install Android.Support.v4 Version 21.0.3., since Xamarin.Forms seemed to be depending on it.
There I got an error saying that it could not be installed since it refers to .NetPortable v4.5.

Do I need Android.Support.v4 and which Framework Version should I choose to create my Solution?
I tried everything from 4.6 down to 4.0.