Wood – the best way to level a tree log

You don't say what saw you used to cut the wood, but if you use a tenon saw you should get a cleaner cut in the first place that requires less sanding. This is because the saw has finer teeth for a smoother cut and a stiffening spine that means you get a straighter cut.

Clamping a second piece of wood to use as a guide is another alternative.

Alternatively you could use a plane to remove the excess wood - but make sure that you clamp a piece of scrap wood to the end of the plank to avoid it splitting:

------> direction of plane
+---------+---+
|         |   |
|    ^    |   | <- Scrap
|    |    |   |
|    |    |   |
  direction of grain

Yeah, that 4" would be a long cut. It could be done in two passes, one from each side. The middle area would have a bit of an overlap and maybe a blade mark, but that could be sanded down. Plus you are probably going to finish it anyways.

What I would recommend though is changing the shape of your transition piece to make it easier to cut, and would also make it more pleasant to walk ok. I posted a similar question regarding a transition from tile to hardwood. Although your question is different, some of the answers there might help you out. For example, take a look at the shape of this transition piece:

In your case, this would be 3 separate cuts instead of one long one. Also the flat top surface will make it more pleasant to walk on vs. the angle (point) you have at the top of your example.