Right-click It.Takes.Two-CODEX.part01.rar and select "Extract Here" using WinRAR or 7-Zip. It will automatically pull data from part04 and all other segments. Mount: Mount the resulting .iso file to a virtual drive. Install: Run setup.exe and follow the prompts.
Only download from trusted trackers or sites to avoid malware. It.Takes.Two-CODEX.part04.rar
Ensure you have all sequential parts (part01, part02, etc.) in the same folder. Right-click It