Why does MySQL crash with the error Error establishing a database connection?

Question

MySQL

After moving to a new vds, the database began to fall off at a load of about 400 simultaneous visitors. Error: Error establishing a database connection . The site is powered by WP.
Server:
Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz 2397.263 Mhz X 4
4 GB memory and 160 GB ssd
Software:
Apache 2.2.15-53.el6.centos
MySQL 5.5.49-1.el6. remi
PHP 5.6.22-1.el6.remi
Perl 5.10.1-141.el6_7.1
Kernel 2.6.32-042stab113.17 Muscle config
:

[mysqld]
pid-file = /var/run/mysqld/mysqld.pid
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
user=mysql
# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0
query-cache-type=1
query-cache-size=64M
max_connections = 1500
thread_cache_size = 40
innodb_buffer_pool_size = 3200M
innodb_flush_log_at_trx_commit = 2
[mysqld_safe]
log-error=/var/log/mysqld.log
pid- file=/var/run/mysqld/mysqld.pid

Answer 1

Well ... the DB drops :) probably due to the load

Answer 2

www.wpbeginner.com/wp-tutorials/how-to-fix-the-err...