Mojave on an MBP4,1 with only 4GB RAM really is ok - but SSD is mission critical (the 4GB RAM stick is certainly another welcome helping - I do not regret the 30bucks ...).
Moving up to an SSD has been the first thing I try to upgrade when I take on a new-to-me Mac. I have one HDD holdout — in a 2013 iMac — because I want to manage all the upgrades (RAM, CPU, SSD) at the same time. Opening it, of course, requires a new adhesive kit.
My routine is to prepare the SSD in an external USB-housing with HFS+ partition-mapping and a small 12GB partition at the very end to hold the MojavePatch-Installer.
Mojave is also my final frontier for the sake of all my favorite 32bit-Apps.
I really like that last part as a means to handle patching updates.