Speedily Read and Parse Data
The part of the code that makes it really slow is this:

string concattedUnitPart = "";
foreach (var line in tempCP4List)
{
  concattedUnitPart = concattedUnitPart + line;
  line1CP4PartLines++;
}


You should not concatenate large strings like that. The string gets longer and longer for each iteration, and it's copied into a new string each time.

If you Read a file that is 1.8 MB, that consists of lines which varies between 50 and 100 characters, you will have been copying about 10 000 MB of data before you have the result.

Also, it scales very badly, so when the files grow it will grow slower at an exponential rate. To handle a file that is 5 MB you will be copying about 80 000 MB of data.

James suggestion to do a replace seems to be a good option. If you want to split and join, you can use the String.Join method:

string[] splitCP4DataBaseLines = CP4DataBaseRTB.Text.Split('\n');
string concattedUnitPart = String.Join(Environment.NewLine, splitCP4DataBaseLines) + Environment.NewLine;