New Firmware with the build number 26.1.A.2.99 has been certified for Xperia M4 aqua over PTCRB. This compares to the previous build number 26.1.A.1.128. So far this update is certified only to the Single SIM version of Xperia M4 Aqua (variants E2303, E2306, E2353).
Xperia M4 Aqua Dual has not certified any new firmware for now. But if this firmware is a major update definitely Xperia M4 Aqua Dual will get it shortly