Now you can programmatically extend StresStimulus functionality and behavior. You can write your own custom logic that can access internal test object model (TOM), generate test parameters, and create other customizations.
1. Scriptable variables. Scriptable variables can be used to create custom request parameters of any complexity. You can create them by writing your own .NET script . Scriptable variables are used like other source variables such as extractors, datasets, functions and data generators. Once the scriptable variable is implemented, it can be used to parameterize requests the same way as the other variables. 
2. Internal scripting capabilities. Now you can create Extensibility objects without any additional developmental tools. You can write your code directly in StresStimulus.
3. Support for three programming languages. You can write your extensibility code in one of the three languages: C#, VB.NET or JScript.NET.
4. External Scriptable Variables. If you wish to create StresStimulus Extensibility objects in Visual studio, you can create external .NET DLLs and register them as External Scriptable Variables.
5. Programming Scriptable Variables. With the StresStimulus interface, you can access internal StresStimulus objects and data from your code. You can access all VU, iteration and request context data such as vu number, iteration number, and request number and values of all extractors. Based on this information, you can create parameters and even modify extractors, that then will be used by StresStimulus.
6. External Components. You create external components that can be used to modify StresStimulus event handling. The following events can be handled by your code: test start / test end, before a request is sent and after a response is received. You can access every VU, iteration and request context data.
To navigate to other parts of the v4.3 release notes, click the links below:
4.5 is available here