Extbase has been delivered as a system extension since TYPO 4.3.0 and it's developed in parallel with TYPO3 FLOW. Version numbers of Extbase and Fluid match the version number of TYPO3 core since 4.6.0.
Although Extbase is available in TYPO3 since version 4.3.0, we recommend that you use the newest possible version - at the very least, 4.5.0. Where performance is critical, then the newest versions of TYPO3 are essential. Fluid templates are compiled to PHP since TYPO3 4.6, which allowed a performance increase of around 200–500%.