I've never used Wavelab 4 so I don't know if the upgrade is worthwhile for you. I got Wavelab 6 with the student discount and it's just incredible for the cost. It's great they take care of students!
Here's the "What's new" listed in the manual:
The main view and overview can independently be set to display one of three modes; Standard waveform, loudness envelope or as a spectrogram
New spectrum editor with high quality linear phase filters that you can either process with surgically or by selected regions.
A rename function that can automatically update all file references in other documents such as Audio Montages, Files, clips and markers can be batch renamed.
Marker regions can be rendered as separate audio files.
Improved file read/write performance and no limitation on audio file size.
DIRAC engine - perhaps the highest quality algorithm for Time stretch/pitch shift processing availiable today.
Crystal Resampler phugin, a professional sample rate converter providing exceptional transparancy and preservation of the high frequency content.
External gear plugin that allows you to process audio via external equipment.
New audio input plugin that replaces the Live input feature to record the input signal.
Option to save an audio selection as an Audio Montage clip.
Redesigned graphic user interface in all menus and dialogs.
Audio split enhancements includes Split to specific file length and Split according to a text file (XML support)
Clip files can be created when using audio split.
The loudness normalizer allows you to specify the desired loudness of a file to a given value. A high quality limiter coupled with multi-pass analysis ensures very accurate results.
The Pan Normalizer allows you to process the left/right channels in an audio file to the same loudness.
The level envelope processor can use an envelope curve.
The Pitch Quantize processor can automatically detect and correct the pitch.
Effect morphing - gradually blend two processed audio ranges with a custom envelope for sample accurate effect transitions.
Global bypass tool that compensates for the loudness difference between processed/unprocessed audio.
New Edit folders dialog manages all default file folders.
Define up to 3 temp file folders to further improve file performanceView setting files (ext ".mem") can store/recall information relating to a specific audio file such as window/zoom/scroll positions, Master section preset etc.
Audio peak files (extension ".gpk") can now be stored in a separate folder.
Improved graphic speed for displaying large files. Peak files can also be created in the background with graphical progress monitoring.
Updated preference dialogs.
Advance "insert silence" functions allows you to insert user specific ambient background noise instead of silence with optional cross-faded insertion.
K-system support for loudness metering.
New Audio Montage tab can mange all plugins used in Montage in a variety of ways. Copy and paste plugins between clips or tracks, or load and save whole plugin chains.
Batch processing of Audio Montage.
Wavelab can now be slaved to the ASIO Positioning Protocol, for sample accurate synchronization.
You can now assign MIDI commands for WaveLab functions.
