How do I get started as a modder?

Hi.
1). Instead of buying a new computer, get an external drive and move everything the sims 4 related there (there are tutorials that show how to move Mods folder). This will be a lot cheaper and easier;
2). Then start learning how to mod. « Modding » is a pretty vague concept in a sense that both a dress, a tattoo and Road to fame can be considered « mods », but they require completely different knowledge, programs and skills. So what you should do is start doing what interests you, if you will end up in a situation where your computer cannot run something you really need, you may consider upgrading.
3). Just in case you do not know, there are windows emulating programs like Wire that will allow you to run windows programs on Mac. If you will get into scripting, there are some minor programs that do not exist for Mac, so it’s good to consider that.
4). Again, I think it’s best to wait a bit before choosing a new machine because yesterday all you needed it for was to play a 9 y o game, today you want to start modding which may involve a lot more demanding programs, next month you may decide to make speed-builds or other videos (processing those requires high CPU), next year you may move to a different game or past gaming at all, so it is best to wait until you know where your current computer is lacking exactly