This first code is to go to form5

private: System::Void btnSend_Click(System::Object^  sender, System::EventArgs^  e) {
             MessageBox::Show("Updated task was sent to project manager.");
             Form5^ f5 = gcnew Form5();
             f5->ShowDialog();
         }

But I thought it would be the same thing of I want to get back to form4 while I'm on form5

private: System::Void btnBack_Click(System::Object^  sender, System::EventArgs^  e) {
     this->Hide();
     Form4^ f4 = gcnew Form4();
     f4->ShowDialog();
 }

But I'm getting a lot of errors, Help me?

To go back to Form4 from Form5 just close Form5 -- you don't need to create a new Form4 because it's already in memory.

private: System::Void btnBack_Click(System::Object^  sender, System::EventArgs^  e) {
     this->Close();
 }