Based on an idea which I came up with in my back yard..
Traveling into the future is a one way trip.. once you're there, there's no way back. Everything travels into the future, but at the same speed relative to all other objects in the universe.
Once you accelerate the speed at which you travel into the future, you go forward in time (it takes you less time to get where other objects are in the future). To the outside observer you appear to move slower and slower multiplied by the acceleration you're experiencing.
You can slow down to the normal speed, and even slower which would cause you to travel slower into the future than all other objects in the universe. To the outside observer you appear to move slower too, the same as with traveling into the future.
The concept of a 4th dimension seems incorrect, since every dimension can travel in 2 ways, back an forth. (3D = up and down, left and right, forward and backwards) Time could be classed as the 3.5th dimension, since you cannot travel backwards in time.
This rules out any strange paradoxes which might destroy the entire universe (as described by Doc Brown in Back to the future (which is a flawed term in itself.. once the future has happened, one cannot go back).
A time machine is something which can manipulate the speed at which time is progressing. Maybe we can achieve effects like this by generating massive gravitational fields which are stronger than the normal gravitational fields found around black holes.