@Celticdode 

The actual speed is affected by many factors: bandwidth you paid for your ISP, the main router, distance, clients,  obstacles/interferences and so on. 

If the actual bandwidth you paid is higher than 1 gigabit, once you order the Deco M4, theoretically, you can get gigabit from the Deco M4 directly; while cause the Deco E4 has the 100Mbps ports, the speed you can get from the E4 by wired cable won't be higher than 100Mbps. 

May it help.