I have ten trees; the wife wants them planted five rows of four to a row
Too simple--plant your 10 trees in the first 2 rows, then take your wife's 10 and plant them in the second 2 rows. Give them plenty of water, and maybe some root stimulator.
I thought this site was full of deep thinkers and problem solvers that could think outside the box.
If you plant a tree at each point of a 5 pointed star and one at each intersecting line that connects the points you will have 5 lines of 4.
Are there prizes involved in this? If you had 12 trees the tress would make a star of David.
Well done we have a winner. It took me 2 weeks to figure it out.