PMBOK4_vs_PMBOK3

I found this table on PMHUB.net that summarizes the difference between the PMBOK 4th and 3rd editions pretty concisely:

PMBOK 3rd ed	PMBOK 4th ed
4.1 Develop Project Charter	4.1 Develop Project Charter
4.2 Develop Preliminary Project Scope Statement
4.3 Develop Project Management Plan	4.2 Develop Project Management Plan
4.4 Direct and Manage Project Execution	4.3 Direct and Manage Project Execution
4.5 Monitor and Control Project Work	4.4 Monitor and Control Project Work
4.6 Integrated Change Control	4.5 Perform Integrated Change Control
4.7 Close Project	4.6 Close Project or Phase
5.1 Scope Planning	5.1 Collect Requirements
5.2 Scope Definition	5.2 Define Scope
5.3 Create WBS	5.3 Create WBS
5.4 Scope Verification	5.4 Verify Scope
5.5 Scope Control	5.5 Control Scope
6.1 Activity Definition	6.1 Define Activities
6.2 Activity Sequencing	6.2 Sequence Activities
6.3 Activity Resource Estimating	6.3 Estimate Activity Resources
6.4 Activity Duration Estimating	6.4 Estimate Activity Durations
6.5 Schedule Development	6.5 Develop Schedule
6.6 Schedule Control	6.6 Control Schedule
7.1 Cost Estimating	7.1 Estimate Costs
7.2 Cost Budgeting	7.2 Determine Budget
7.3 Cost Control	7.3 Control Costs
8.1 Quality Planning	8.1 Plan Quality
8.2 Perform Quality Assurance	8.2 Perform Quality Assurance
8.3 Perform Quality Control	8.3 Perform Quality Control
9.1 Human Resource Planning	9.1 Develop Human Resource Plan
9.2 Acquire Project Team	9.2 Acquire Project Team
9.3 Develop Project Team	9.3 Develop Project Team
9.4 Manage Project Team	9.4 Manage Project Team
10.1 Communications Planning	10.1 Identify Stakeholders
10.2 Information Distribution	10.2 Plan Communications
10.3 Performance Reporting	10.3 Distribute Information
10.4 Manage Stakeholders	10.4 Manage Stakeholder Expectations
	10.5 Report Performance
11.1 Risk Management Planning	11.1 Plan Risk Management
11.2 Risk Identification	11.2 Identify Risks
11.3 Qualitative Risk Analysis	11.3 Perform Qualitative Risk Analysis
11 .4 Quantitative Risk AnalysiS	11.4 Perform Quantitative Risk Analysis
11.5 Risk Response Planning	11.5 Plan Risk Responses
11.6 Risk Monitoring and Control	11.6 Monitor and Control Risks
12.1 Plan Purchases and Acquisitions	12.1 Plan Procurements
12.2 Plan Contracting	12.2 Conduct Procurements
12.3 Request Seller Responses	12.3 Administer Procurements
12.4 Select Sellers	12.4 Close Procurements
12.5 Contract Administration
12.6 Contract Closure

Additional appendix on interpersonal skills

Leadership
Teambuilding
Motivation
Communication
Influencing
Decisionmaking
Political and Cultural Awareness
Negotiation

The major differences between the Third Edition and the Fourth Edition are summarized below in PMI's webpage:

All process names are now in a verb-noun format

Efforts were made to distinguish between Enterprise Environmental Factors and Organizational Process Assets.
A standard approach for discussing requested changes, preventive actions, corrective actions and defect repairs was employed.
The processes decreased from 44 to 42:

Two processes were deleted.
Two processes were added.
Six processes were reconfigured into 4 processes in the Procurement knowledge area.
To provide clarity, a distinction was made between the Project Management Plan and project documents used to manage the project.
The distinction between the information in the Project Charter and the Project Scope Statement was clarified.
The process flow diagrams at the beginning of chapters 4-12 have been deleted and replaced with data flow diagrams.
A data flow diagram for each process has been created to show where information comes from as an input and where it goes to as an output.
A new appendix was added that addresses key interpersonal skills that a project manager utilizes when managing a project.

PMI also notes that a complete list of changes can be found in Appendix A of the new PMBOK Guide-Fourth Edition.