It looks like someone is desperately trying to convey something that they don't want anyone else to know.
Try to crack the code and give the answer as a four-letter word.
Imagine that the zigzag line from the top-left to the bottom-right, going down, across and down, is one continuous line.
Continuing on from hint one...
If you were to remove the line, you would see four symbols.
Think of the line discussed in hint one as a mirror.
The code is written in mirror writing.
Cut each of the four symbols in half to see which letters they make.
Answer for 'Mystery Letter'
The answer is 'LOVE'.