Apple could host an event on April 4 given that was the date Apple’s online store listed for in-store availability of 12.9-inch iPad Pro models in the United States at the time. The rumor is likely false.
Apple’s in-store Personal Pickup tool is generally not a reliable indicator of a specific launch date or Apple event. The in-store pickup date for 12.9-inch iPad Pro models is a rolling target, as evidenced by the April 5.
2.9-inch iPad Pro’s current 2-3 week shipping estimate, plus a few extra days to reach the store. On February 9, for example, we reported that most 12.9-inch iPad Pro models were available for in-store pickup on March 3 at the time.
Apple could still hold an event on April 4. In January, however, Apple said it had an issue with one of its iPad suppliers that was not expected to be fully resolved this quarter, which could reasonably explain the shortage.
Apple is expected to launch at least a trio of new iPad Pro models in the near future, though, so an event might be announced soon enough. Japanese website Mac Otakara said Apple is planning a March event where it will unveil those new iPads alongside a 128GB iPhone SE, red iPhone 7, and new Apple Watch bands.