Should we expext R4 (ram optimized) or M4 (balanced) instances soon or is the high cost of DDR4 going to delay things? I am about to reserve some R3 instances

Should we expext R4 (ram optimized) or M4 (balanced) instances soon or is the high cost of DDR4 going to delay things? I am about to reserve some R3 instances so maybe that will speed it up :-)