This issue raise because the memory > limit_memory_soft. I think your server Memory is 4GB so you got this error.

You can calculate the  limit_memory_soft and  limit_memory_hard depend on workers and workers depend on the concurrent users.

For Example:

Server with 4 CPU Core, 8G RAM

60 concurrent users
60 users / 6 = 10 workers = (core * 2) + 1 = (4*2)+1(Cron Worker) so (4 * 2) + 1 = 9
RAM = 9 * ((0.8*150) + (0.2*1024)) ~= 3Go RAM for Odoo

limit_memory_hard = 768 * 9  (Workers) * 1024 * 1024 = 7247757312

limit_memory_soft = 640 * 9 (Workers)* 1024 * 1024 = 6039797760

max_cron_threads = 1

workers = 8

Note: You need to restart Odoo after any change in config file.

Multiprocessing options:
    --workers=WORKERS   Specify the number of workers, 0 disable prefork mode.
    --limit-memory-soft=LIMIT_MEMORY_SOFT
                        Maximum allowed virtual memory per worker, when
                        reached the worker be reset after the current request
                        (default 671088640 aka 640MB).
    --limit-memory-hard=LIMIT_MEMORY_HARD
                        Maximum allowed virtual memory per worker, when
                        reached, any memory allocation will fail (default
                        805306368 aka 768MB).
    --limit-time-cpu=LIMIT_TIME_CPU
                        Maximum allowed CPU time per request (default 60).
    --limit-time-real=LIMIT_TIME_REAL
                        Maximum allowed Real time per request (default 120).
    --limit-request=LIMIT_REQUEST
                        Maximum number of request to be processed per worker
                        (default 8192).