C# 4, or C# version 4.0, was released on April 10, 2010.
Here are books about C# 4.
2010-02-03
C# 4.0 In A Nutshell
Edition: 1
Series: The Definitive Reference
Authors: Joseph Albahari and Ben Albahari
Publisher: O’Reilly
ISBN: 9780596800956
Type: Book
Format: Paperback
Price: US $50
On sale @ Amazon.
Shop @ O’Reilly.
2010-03-10
Essential C# 4.0
Edition: 3
Series: Microsoft .NET Development
Author: Mark Michaelis
Publisher: Pearson
ISBN: 9780321694690
Type: Book
Format: Paperback
Price: US $60
On sale @ Amazon.
Shop @ Pearson.
2010-08-18
Programming C# 4.0
Edition: 1
Series: Programming
Authors: Ian Griffiths, Matthew Adams, and Jesse Liberty
Publisher: O’Reilly
ISBN: 9780596159832
Type: Book
Format: Paperback
Price: US $55
On sale @ Amazon.
Shop @ O’Reilly.
2010-08-31
C# 4.0
Edition: 1
Series: Pocket Reference
Authors: Joseph Albahari and Ben Albahari
Publisher: O’Reilly
ISBN: 9781449394011
Type: Book
Format: Paperback
Price: US $15
On sale @ Amazon.
Shop @ O’Reilly.
2010-10-31
The C# Programming Language
Edition: 4
Series: Microsoft .NET Development
Author: Anders Hejlsberg, Mads Torgersen, Scott Wiltamuth, and Peter Golde
Publisher: Pearson
ISBN: 9780321741769
Type: Book
Format: Hardcover
Price: US $40
On sale @ Amazon.
Shop @ Pearson.
2010-11-25
C#
Edition: 2
Series: In Depth
Author: Jon Skeet
Publisher: Manning
ISBN: 9781935182474
Type: Book
Format: Paperback
Price: US $50
On sale @ Amazon.
Shop @ Manning.
P4TE 