The ICs in U58 & U59 are the CPU RAM.
"MT4C16257DJ" stands for Micron Technology DRAM, 256k x 16 EDO, 5.0V RAM-chips with a capacity of 256k x 16 Bit = 4 megabit = 512 kilobyte
"C" indicates that a 5.0V power supply is required.
"DJ" indicates the IC-type: 40-pin SOJ
As written in the manual e64, EIV, e6400 and E4K are equipped with just 1MB of CPU RAM while E4X and E-Synth come with 4MB. That's 4 times more memory.
Therefore E4X & E-Synth use 2x 2MB RAMs instead of 2x 512KB RAMs like in e64, EIV, E4K and e6400.
E4X & E-Synth maybe use RAMs of the type "1024k x 16" respectively "1Meg x 16".
Micron once manufactured RAMs of the type MT4C1M16EDJ with 42-pins.
Closeup pictures of the area around U58 & U59 would be very helpful. (Hoping that an angels descends from heaven and provides the pictures...
)
You sure recognized that the MT4C1M16EDJ RAMs have 2 pins more, that are required for adressing of higher memory areas.
The jumpers on the right hand side configure how these ICs are treated: as 1Meg x 16 or 256k x 16 RAMs
If you have "1Meg x 16" RAMs installed then you have to set both jumpers to the 1M-position to (I think) re-route address lines for accessing the higher memory areas.
An upgrade is only possible if you unsolder/remove the installed RAMs in U58 & U59 and replace them with two MT4C1M16EDJ, if you can get them somewhere and get to know how the RAMs with the two additional pins are installed.