Any upgrade to 4.x will require a new license. Reference.
You can go up to 3.2.2 SP4 with your current license. The 3.x release train will limit you as 4.x is required for newer devices and software (e.g. ASA 8.3 or higher). Also, 4.x is a much better application - new and improved features, 64-bit application, etc.. (Features by release)
