Why does LTE appear instead of 4G network ??

iPhones marketed for different regions may differ in whether they display 4G or LTE when connected to an LTE network. In addition, phones sold for the US market (which display LTE) may display 4G when connected to an HSPA+ network. LTE and HSPA+ are both "4G" networks, though LTE is considerably faster.

LTE is a form of 4G specifically 4G Long Term Evolution. This link might help explain it in more detail.

https://www.tccrocks.com/blog/what-is-difference-between-4g-and-4g-lte/