More eggs are likely imminent as the pair continue to breed. You'll notice that the egg is not being continually incubated. This is not a cause for alarm. Incubation is often delayed until the last egg is laid. This may be a strategy to reduce the time between hatching for the eggs. Eggs are incubated, on average, for 33 days. Four eggs is a typical clutch, although 5 or even 6 occurs rarely.
For more 1st egg action be sure to check out the Norfolk Botanical Garden Eagle Cam - where the 1st egg of a replacement clutch was laid on Sunday.