Highlight the cells from the second one onwards - i.e. from A2 in your
example. Click Format | Conditional Format and in the pop-up select
Formula Is rather than Cell Contents and in the next panel enter this
formula:
A2=A1+1
Click the Format button on the panel, click patterns (background
colour) and choose your highlight colour, eg Bright Yellow. Then click
OK twice and you should see the number 4 highlighted with bright
yellow.
Hope this helps.
Pete