First things first, I could do the math in my head, but I want to make this a step by step process. The second thing is, unpwnd doesn't require a specific size so the images in this walkthru will not be 620 px. That is not lost on me, I happen to Blogger as my platform and it has very different tools from Wordpress.
Height is another story. I am going to take away a multiple of 620. I decided that I would go with 620 times 2, so what I end up with is 2760. That is 4000-1240=2760.
Before I hit resize, I clicked the Center button. It just so happens that my subjects are dead center and this works. If it did not, I could have adjusted this manually.
Anyway, I think I can trim a little more of the edges and when I do that, I want my height and width to be an exact multiple of 620. I take the width of 2990 and divide by 620 which gives me 4.822 and some change. I do the same for the height which gives my 4.451 and change.
Now for a trick. I am going to take four away from each number leaving 0.822 and 0.451. Both of these numbers need to multiplied by 620. 620 is the only number I know for this process, which is why it keeps popping up. The results are 510 and 280.
2990-510 is 2480 and 2760-280 is also 2480. 2480 divided by 620 is 4.
Again, I am using the center bottom so I don't clip away my subjects. Also, this could a manual process.
And here is the final output at 620 px.