ScrollArea in QT Issue?

I have one form where i am applying ScrollArea but it is not getting applied properly ? I want to place the four buttons in Scrollable Area and when buttons gets increased a scroll bar should appear there to scorll down vertically.

Currently when i am trying to add more buttons the buttons overlap with the combobox and does not grow downward.

Here is my code :

 QPushButton *b1 = new QPushButton(strsearch);
           QPushButton *b2 = new QPushButton(strsearch);
           QPushButton *b3 = new QPushButton(strsearch);
           QPushButton *b4 = new QPushButton(strsearch);

 b1->setStyleSheet(

               "background-image: url(:/user.gif);"
               "background-repeat:no-repeat;"
               "background-attachment:fixed;"
               "background-position:left top;"
  开发者_高级运维             "border-style: outset;"
               "background-color : black;"
               "border-width: 2px;"
               "border-radius: 10px;"
               "border-color: black;"
               "font: bold 16px;"
               "color: black;"
               "min-width: 10em;"
               "min-height: 0.75em;"
               "margin: 0 1px 0 1px;"
               "color:rgb(255,255,255);"
               "padding: 6px;"
               );



b2->setIcon(QIcon(":/user.gif"));
           b2->setIconSize(QSize(160, 26));
           b3->setIcon(QIcon(":/user.gif"));
           b3->setIconSize(QSize(160, 26));
           b4->setIcon(QIcon(":/user.gif"));
           b4->setIconSize(QSize(160, 26));

 QGridLayout *layout = new QGridLayout;
 layout->addWidget(b1, 1, 0);
           layout->addWidget(b2, 2, 0);
            layout->addWidget(b3, 3, 0);
            layout->addWidget(b4, 4, 0);
layout->addWidget(scrollArea);
  layout->setAlignment(Qt::AlignBottom);
  setLayout(layout);

//Create the buttons
QPushButton *b1 = new QPushButton("Button 1");
QPushButton *b2 = new QPushButton("Button 2");
QPushButton *b3 = new QPushButton("Button 3");
QPushButton *b4 = new QPushButton("Button 4");

//Add the buttons to a vertical layout (faster than grid layout)
QVBoxLayout *scrollLayout = new QVBoxLayout;
scrollLayout->addWidget(b1);
scrollLayout->addWidget(b2);
scrollLayout->addWidget(b3);
scrollLayout->addWidget(b4);

//Create a viewport widget that contains the layout with buttons
QWidget *viewport = new QWidget;
viewport->setLayout(scrollLayout);

//Add the viewport to the scroll area
QScrollArea *scrollArea = new QScrollArea;
scrollArea->setWidget(viewport);

//Add the scroll area to your main window's layout
mainLayout->addWidget(scrollArea);