短く書けばこれだ。
<widget class="QTabWidget" name="tabWidget_1">
<property name="currentIndex">
<number>0</number>
</property>
<widget class="QWidget" name="tab_3">
<attribute name="title">
<string>Tab 1</string>
</attribute>
<layout class="QGridLayout" name="gridLayout_4">
<item row="0" column="0">
<widget class="QDateTimeEdit" name="dateTimeEdit_2"/>
</item>
<item row="1" column="0">
<widget class="QDateTimeEdit" name="dateTimeEdit"/>
</item>
</layout>
</widget>
<widget class="QWidget" name="tab_4">
<attribute name="title">
<string>Tab 2</string>
</attribute>
<layout class="QGridLayout" name="gridLayout_5">
<item row="0" column="0">
<widget class="QCheckBox" name="checkBox_2">
<property name="text">
<string>CheckBox</string>
</property>
</widget>
</item>
<item row="1" column="0">
<widget class="QCheckBox" name="checkBox">
<property name="text">
<string>CheckBox</string>
</property>
</widget>
</item>
</layout>
</widget>
</widget>ファイルとして成立させるならこうだ。
<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
<class>MainWindow</class>
<widget class="QMainWindow" name="MainWindow">
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>1146</width>
<height>794</height>
</rect>
</property>
<property name="windowTitle">
<string>MainWindow</string>
</property>
<widget class="QWidget" name="centralwidget">
<layout class="QVBoxLayout" name="verticalLayout_2">
<item>
<layout class="QVBoxLayout" name="verticalLayout">
<item>
<layout class="QHBoxLayout" name="horizontalLayout">
<item>
<widget class="QGroupBox" name="groupBox">
<property name="title">
<string>GroupBox</string>
</property>
<layout class="QVBoxLayout" name="verticalLayout_9">
<item>
<layout class="QVBoxLayout" name="verticalLayout_6">
<item>
<widget class="QLabel" name="label_3">
<property name="text">
<string>TextLabel</string>
</property>
</widget>
</item>
<item>
<widget class="QLabel" name="label_4">
<property name="text">
<string>TextLabel</string>
</property>
</widget>
</item>
</layout>
</item>
</layout>
</widget>
</item>
</layout>
</item>
<item>
<layout class="QHBoxLayout" name="horizontalLayout_2">
<item>
<widget class="QGroupBox" name="groupBox_2">
<property name="title">
<string>GroupBox</string>
</property>
<layout class="QVBoxLayout" name="verticalLayout_8">
<item>
<layout class="QVBoxLayout" name="verticalLayout_7">
<item>
<widget class="QLabel" name="label">
<property name="text">
<string>TextLabel</string>
</property>
</widget>
</item>
<item>
<widget class="QLabel" name="label_2">
<property name="text">
<string>TextLabel</string>
</property>
</widget>
</item>
</layout>
</item>
</layout>
</widget>
</item>
</layout>
</item>
<item>
<layout class="QHBoxLayout" name="horizontalLayout_3">
<item>
<widget class="QGroupBox" name="groupBox_3">
<property name="title">
<string>GroupBox</string>
</property>
<layout class="QVBoxLayout" name="verticalLayout_3">
<item>
<layout class="QVBoxLayout" name="verticalLayout_4">
<item>
<widget class="QLabel" name="label_6">
<property name="text">
<string>TextLabel</string>
</property>
</widget>
</item>
<item>
<layout class="QVBoxLayout" name="verticalLayout_5"/>
</item>
<item>
<widget class="QTabWidget" name="tabWidget_1">
<property name="currentIndex">
<number>0</number>
</property>
<widget class="QWidget" name="tab_3">
<attribute name="title">
<string>Tab 1</string>
</attribute>
<layout class="QGridLayout" name="gridLayout_4">
<item row="0" column="0">
<widget class="QDateTimeEdit" name="dateTimeEdit_2"/>
</item>
<item row="1" column="0">
<widget class="QDateTimeEdit" name="dateTimeEdit"/>
</item>
</layout>
</widget>
<widget class="QWidget" name="tab_4">
<attribute name="title">
<string>Tab 2</string>
</attribute>
<layout class="QGridLayout" name="gridLayout_5">
<item row="0" column="0">
<widget class="QCheckBox" name="checkBox_2">
<property name="text">
<string>CheckBox</string>
</property>
</widget>
</item>
<item row="1" column="0">
<widget class="QCheckBox" name="checkBox">
<property name="text">
<string>CheckBox</string>
</property>
</widget>
</item>
</layout>
</widget>
</widget>
</item>
<item>
<widget class="QLabel" name="label_5">
<property name="text">
<string>TextLabel</string>
</property>
</widget>
</item>
</layout>
</item>
</layout>
</widget>
</item>
</layout>
</item>
</layout>
</item>
</layout>
</widget>
<widget class="QMenuBar" name="menubar">
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>1146</width>
<height>26</height>
</rect>
</property>
</widget>
<widget class="QStatusBar" name="statusbar"/>
</widget>
<resources/>
<connections/>
</ui>
元ネタはこちら
https://stackoverflow.com/questions/6221551/qtabwidget-refusing-to-auto-resize