To upgrade from v4.1.2 you need to apply patches in this specific order:
4.1.5 >> 188.8.131.52 >> 184.108.40.206 >> 4.1.6 patch
Then, you can upgrade from v4.1.6 to v4.5 using the 4.5 Upgrade script. Or, you can install a clean 4.5 website and import your 4.1.6 backup using 4.1.6 Database Importer plug-in.
Finally, you can install the latest 4.5.1 patch.
You can find all patches in your Client Area
with necessary instructions on performing careless upgrade. If anything goes wrong during the upgrade, feel free to contact the support staff