In IC4, the file size limit was constrained by the int(11) field in the database, which has a maximum value of 2,147,483,647 bytes (~2GB). This is because the int type is a 32-bit signed integer.
In IC5, Invision upgraded the relevant database fields to bigint(20), allowing for a maximum file size of 9,223,372,036,854,775,807 bytes (~8EB). This change means the file size limit on the database side is no longer a bottleneck.
(At least that is what I understand)