You have to use a different version of GP4.exe temporarily for GP4Master and then put the old one back that you play GP4 with. I can email it to you to get GP4Master to work. All you have to do is copy or rename your current GP4.exe (say GP4p.exe) p for playing. Then insert the new GP4.exe in place of your current one. GP4Master will accept it. Convert the tga to a tex and shut GP4Master. Delete the GP4.exe and rename/replace that one with your original.
If you want me to email you the GP4Master friendly exe or need more help let me know.