Is Avengers 4 the last Avengers movie?
It is the last confirmed Avengers film. Given the success of the MCU, it is certain to continue, and another big team up will happen. It will probably be a quite different set of Avengers to the ones we saw in first two.
If they want to go back to six core Avengers, it might include Spider Man, Captain Marvel, Dr Strange, Black Panther, and Ant-Man and/or Wasp. A Black Widow film is in the works. This is set to be a prequel, so there is no guarantee she survives Avengers 4, but if she does she will probably still be an Avenger.
No; this is not a last movie of avengers but for some heroes like Captain America and might be Tony it can be last. After this marvel will work for new superheros. Already marvel is working for marvel future fight as per sources.