1.- it will be compatible with vB4 very soon. We are workin now on vBET 4.0.0 version which will be compatible with vB4 - it will be released at the beginning of the march. In next releases we will make it compatible with vBSEO3.5 and this will also happen soon. making vBET 4.x compatible with vbseo-sitemap-generator is also in plans. We didn't make yet any plans about integration with vbadvanced4.
2.- it translated content of outhput html. So if other plugin affects output then ti will be translated. If it is made by Ajax then by default i will not be translated (if it is not integrated with vBET) because it is not part of output html. You can try free version first and see it in live. Here it is for vB4: vB Enterprise Translator - vBulletin.org Forum
3.- We didn't test it. If rewriting is working as vB hook and it happens before global_complete or in global_complete with execution order less than 1000 then it will be translated.