eu4 = continuous effort over many centhuries to lead, transform and expand your country while paying attention to many aspects of it other than military, such as diplomacy, trade, economy and religion. a lot to focus on, a very wide range of possible goals, hundreds of small wars over a long campaign and usually very rarely large ones. essentially, long and deep but not necessarily intense focus on very rich and in-depth system of mechanics in order to continuously lead your country to whatever goal you have in mind.
hoi4 = 3-4 hours of intense preparation of war, 1-2 hours of very intense focus on conducting war and it's over. peace conference screen for most major nations is when you can already pat yourself on the back or slap yourself on the forehead depending on how the game went
the game is all about ww2, ww2 is what you will be preparing for and then what you will be fighting, and then it's done - very different from eu4 where it's a multitude of small goals over long period of time
based on this you can choose what fits you the most. obviously i encourage you to eventually buy both games, but if there's someone who understands financial limitations it's me, so just think about what you're looking forward to more and i hope my short elaboration on the concept of each game is going to prove useful as you consider your options
one more thing to consider - eu4 is mostly about SP, hoi4 is mostly about MP