There are still five trains. The ride can only run four, the extra train is so the ride can run at full capacity while a train is being overhauled. Kumba also has an extra train (four, can run three), and I think Montu does as well.
Unfortunatly United Parks newer rides (especially at Seaworld Orlando) do not have an extra train, so the rides cannot run at full capacity during train overhaul. (Cost cutting measure.) This can have major operational impacts if an overhaul takes longer than expected.