 Here is the Python code for the `SelectableGrid` class and its usage:

```python
from kivy.uix.focusbehavior import FocusBehavior
from kivy.uix.compoundselectionbehavior import CompoundSelectionBehavior
from kivy.uix.gridlayout import GridLayout
from kivy.core.window import Window
from kivy.uix.button import Button
from kivy.uix.label import Label
from kivy.properties import NumericProperty, ListProperty, ObjectProperty

class SelectableGrid(FocusBehavior, CompoundSelectionBehavior, GridLayout):
    selected_nodes = ListProperty([])

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.bind(minimum_height=self.setter('height'), size_hint_y=None)

    def on_key_down(self, widget, keycode):
        if keycode[1] == 'number':
            index = int(keycode[0]) - 1
            if 0 <= index < len(self.children):
                self.children[index].focus = True

    def on_key_up(self, widget, keycode):
        if keycode[1] == 'number':
            self.select_with_keycode(keycode)

    def select_with_keycode(self, keycode):
        index = int(keycode[0]) - 1
        if 0 <= index < len(self.children):
            node = self.children[index]
            if node not in self.selected_nodes:
                self.select_node(node)
            else:
                self.deselect_node(node)

    def select_node(self, node):
        node.background_color = (1, 1, 1, 1)  # RGBA for white
        self.selected_nodes.append(node)
        print(f'Selected node: {node}')

    def deselect_node(self, node):
        node.background_color = (1, 1, 1, 0)  # RGBA for transparent white
        self.selected_nodes.remove(node)

    def on_touch_down(self, touch):
        if self.collide_point(*touch.pos):
            for child in self.children:
                if child.collide_point(*touch.pos):
                    child.focus = True
                    break

    def on_touch_move(self, touch):
        for child in self.children:
            if child.focus:
                child.focus = child.collide_point(*touch.pos)

    def on_touch_up(self, touch):
        for child in self.children:
            if child.focus:
                self.select_node(child)
                child.focus = False

grid = SelectableGrid(cols=4, row_force_default=True, row_default_height=50)

for i in range(40):
    button = Button(text=str(i + 1), size_hint_y=None, height=grid.row_default_height)
    grid.add_widget(button)
    button.bind(touch_down=grid.on_touch_down)

root = grid
```

This code creates a `SelectableGrid` class that inherits from `FocusBehavior`, `CompoundSelectionBehavior`, and `GridLayout`. The class handles key down and key up events, navigates to a node by typing its number, selects and deselects nodes, and handles touch events. The selection of a node changes its background color, and the selected nodes are printed when the selection changes.

An instance of the `SelectableGrid` class is created with specific parameters, and 40 buttons are added to it, each with a unique number as its text. The touch down event of each button is bound to the touch handling method of the grid. Finally, the application is run with the grid as the root widget.