Why is MariaDB dropping multiple connections?

Question

Java

Please help.
When connecting from java to mairadb, I get the error "could not create connection to database server", and this happens when the number of requests increases. And initially everything is fine. The server is designed in such a way that when it is rebooted, devices are connected to it, there are > thousands of them in total, and at some stage I get this error. Various max_connection and other - exposed - nothing helps.
If there were problems with table locks, I would see other errors.
I ran two analyzers, adjusted everything according to their recommendations.

my my.cnf

[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
connect_timeout = 2
max_connections = 4000
open_files_limit = 16384
innodb_open_files = 16384
max_allowed_packet = 64M
slow_query_log = 1
long_query_time = 1
slow_query_log_file = /var/log/mysql/slow-query.log
log_queries_not_using_indexes
thread_cache_size = 1500
concurrent_insert=ALWAYS
low_priority_updates=1
query_cache_size = 0
query_cache_type = 0
innodb_stats_on_metadata = OFF
innodb_file_per_table=ON
skip-name-resolve

What parameters to look at?
Thanks to those who responded for help