Answer based on context:

The war had three phases. Initially it was a localized feud between supporters of Gebhard and those of the Catholic core of the Cathedral Chapter. With the election of Ernst of Bavaria as a competing archbishop, what had been a local conflict expanded in scale: Ernst's election guaranteed the military, diplomatic, and financial interest of the Wittelsbach family in the Electorate of Cologne's local affairs. After the deaths of Louis VI, Elector Palatine in 1583 and William the Silent in 1584, the conflict shifted gears again, as the two evenly matched combatants sought outside assistance to break the stalemate. Finally, the intervention of Alexander Farnese, Duke of Parma, who had at his command the Spanish Army of Flanders, threw the balance of power in favor of the Catholic side. By 1588, Spanish forces had pushed Gebhard from the Electorate. In 1588 he took refuge in Strassburg, and the remaining Protestant strongholds of the Electorate fell to Parma's forces in 1589.

How many years after William the Silent did the Spanish forces push Gebhard from the Electorate?
4