Is it possible to update BQ aquaris E4.5 with latest firmware for BQ aquaris A4.5
If the specs from PDAdb is to be believed, the 2 phones are quite different hardware-wise:

BQ Aquaris E4.5 Dual SIM vs. BQ Aquaris A4.5 4G LTE Dual SIM

The A4.5 is a relatively new Android One phone that's promised 2 years of support. The E4.5 on the other hand, despite released in late 2014, bears a MT6582 SoC which had fallen out of mainstream for quite some time. There's no way you could cross-flash between these two, and you likely won't find anything tailored to this particular device either.