Laptop upgrade

@infinitespectral  If you're satisfied with how your Mac runs Sims 3 and are eventually going to build a gaming desktop, then you could certainly spend a lot less on a laptop for Sims 4, maybe somewhere around $500.  In this range, all the options have integrated graphics (a chip integrated into the processor), so the only question is how good that processor's graphics chip is.

To that end, a laptop with an i5-1135G7 and at least 256 GB storage is your best bet.  If you saw an i7-1165G7, that would be even better, but those often come in more expensive laptops, and while Sims 4 would benefit from that extra power, the difference probably isn't worth an extra $200, especially when that $200 would put you in range of a proper gaming laptop.  256 GB is enough storage for Sims 4 and all your content, plus however many smaller apps you want to install, but you might want 512 GB just so you don't need to worry about it in the future (or if you want to install other games).

I have no idea what your local Microcenter will have in stock, but you can look over the options with those specs and pick something that works for you.  Before buying, google the exact model and the word cooling; if you see any red flags, pick something else.  And if you see a specific model and would like a second opinion, feel free to link it here.

Again, it's possible that a laptop with one of these processors would also run Sims 3 perfectly well, but it's not guaranteed.  So feel free to experiment if you want; just be prepared to go back to your Mac for Sims 3.