PHP - Regular expression
replace grater than 4 digit character to given character using PHP

$original_string = "test343443test34demo55555"

$replace_string = "TEST"

output
------
$output = testTESTtest34demoTEST

How do I do this using PHP?
PHP - Regular expression
===

replace grater than 4 digit character to given character using PHP

$original_string = "test343443test34demo55555"

$replace_string = "TEST"

output
------
$output = testTESTtest34demoTEST

How do I do this using PHP?