Are four vectors in Special Relativity considered to be tensors?

Question

In particular, I would like to know if the four velocity and the four acceleration are tensors.

Answer 1

"Tensors" are pretty general. Scalars are (rank 0) tensors. Vectors are (rank 1) tensors, including 4 vectors. So are matrices, etc.

Answer 2

The four velocity and four acceleration are vectors in a space-time Minkowski space and can be represented as a quadruplet $(u_0,u_1,u_2,u_3)$ and $(a_0,a_1,a_2,a_3)$ where the elements with subscripts $1-3$ represent the ordinary Euclidian vectors for velocity and acceleration, in proper time.

They are not tensors (of rank two), an example of which is the Electromagnetic Field tensor.