Several new object types are supported now. This allows to control test execution flow and closer emulate real user activity. You can create New object types at any location on the test case tree.
1. Rendezvous. Rendezvous are points in a test case that are used to synchronize Virtual Users so that they carry out tasks at the same moment. This is used to order to create a simultaneous load on the server at specific points in the test case. 
2. Delays. A delay feature was added which adds a pause in iteration execution. You can configure the duration of the delay.
3. Skip to Next. The skip to next iteration feature stops the current iteration execution. This might be useful if the test case has a fail condition and should retry by starting the test case iteration over again.
4. Simplified Loops. In the previous versions, loop implementation was overly complicated. In 4.5 the following simplifications are provided: Unconditional loops are now called Loops. Conditional loops, now called Do...While, handle a group of conditions to continue or stop looping. The other group of conditions to run or skip the loop is now handled by If...then objects. This change is backward-compatible and pre 4.5 scripts are fully supported.
5. If..Then. An If...Then condition will execute its child objects if a given condition is true (or false). The condition will compare an extractor to a string to determine if the loop should run.
6. Do..While Conditional Loops. Conditional loops have a condition which is checked at the end of the loop to determine if the loop should continue or exit. You can configure a loop to exit when the condition returns true or false.
To navigate to other parts of the v4.3 release notes, click the links below:
4.5 is available here