J
In your described situation, you've got a 20% chance of surviving the teleport.
From the same line in the wiki you outlined:
Pressing the action button while carrying one will teleport you in the direction you are facing. You will appear a random number of tiles away in the direction you are holding, either 4-8 tiles horizontally or upwards, or 5-9 tiles downwards.
If the destination tile is a solid block, the teleporter tries to find an empty space up to 3 tiles above the selected location - If there is no viable space, the teleporter will warp you into a solid block and you will be killed.
The teleporter does two checks when you use it, the first is picking the space in line with where you use the item; this check does not take into account whether or not you'll survive the teleport. After it's chosen how far you'll travel, then the second check will check the randomly selected space and the three spaces above; if the original space is open, you'll go there, otherwise you teleport to the first open space above. If none of the spaces above are open, then you teleport to the original space and die.
So this situation gives you a 20% chance of survival, where the initial check landing in columns 4, 5, 7, or 8 would mean instant death.
0 1 2 3 4 5 6 7 8 9
0 [ ][X][X][X][X][X][X][X][X][X]
1 [ ][X][X][X][X][X][ ][X][X][X]
2 [ ][X][X][X][X][X][ ][X][X][X]
3 [ ][X][X][X][X][X][ ][X][X][X]
4 [ ][X][X][X][X][X][ ][X][X][X]
5 [P][X][X][X][X][X][ ][X][X][X]
6 [X][X][X][X][X][X][X][X][X][X]
To contrast this, you'd survive this teleport every single time you tried it.
0 1 2 3 4 5 6 7 8 9
0 [ ][X][X][X][X][X][X][X][X][X]
1 [ ][X][X][X][X][X][X][X][X][X]
2 [ ][X][X][X][X][X][X][X][X][X]
3 [ ][X][X][X][ ][ ][ ][ ][ ][X]
4 [ ][X][X][X][X][X][X][X][X][X]
5 [P][X][X][X][X][X][X][X][X][X]
6 [X][X][X][X][X][X][X][X][X][X]