The standard maximum RAM on a Model 4 was 128 KB. HyperMEM expands that up to a megabyte with -mem hyper:1024 and is represented as a larger RAM in the debugger, recording and trace. For memory sizes greater than 128 K hyperMEM is assumed. -mem 256 is equivalent to -mem hyper:256 on a Model 4.
The Model 4 also features two expansion RAMs, -mem super:1024 and -mem mega:16384. These don't extend main memory but provide a new tranche of banked RAM. The Record menu and the debugger refer to them as "Expansion RAM" and accesses to it show up as expram[] in the trace log.
The Model 4 can have an additional 256K with the XLR8er mod installed via the -z180 flag. The Record menu and the debugger refer to them as "XLR8er RAM" and accesses to it show up as xlr8er_ram[] in the trace log.