If you see my first two links you'll see from the astronauts point of view they can accelerate indefinitely with no penalty besides chance of colliding with mat

If you see my first two links you'll see from the astronauts point of view they can accelerate indefinitely with no penalty besides chance of colliding with matter on the way. Accelerating at 1g for two years and then decelerating at 1g for two years means you will travel 3 light years in 4 years. Acceleration beyond 1g will speed this up from the astronauts point of view. At 2g it would take only 2.5ish years to reach alpha centauri from the travellers reference.