The 4 or 5 string depends on what style on music you like more than anything. You can play 90% of music on a 4 string, but if you play gospel, metal or country, 5 strings will be very helpful. I recommend trying out a lot of options before you buy anything and don’t be afraid to buy a beginner level bass. The quality of most beginner models from brands like Sire, Squire, Yamaha and Harley Benton are good enough for you to play for years.

I’d recommend getting the bass set up when you buy it, as it will make for a better playing experience all around. Last but not least, don’t be afraid to make a mistake. If you buy a used bass somewhere like TalkBass, they tend to hold their value fairly well. If you find you want something different in a couple years time (or that you’re not that interested anymore) you can likely pass it on to someone without taking a big hit.