Sign the 2* player and release the 4* one...
If you have plenty on tokens then you can sign the 4* it depends upon you...But why sign a 4* player when you can get a 5* player from the market and in much less tokens...
You can also sing him if you are short in cash and do not have the money to buy a 5* player...
With the 2 star keep him sell him at the start of the season in the mean time bring him close to 1 skill point of being a scout for levels lower than yours and after that start teaching him special ability...