3 or 4 season tent refers mainly to its ability to handle snow loading. If you get lots of snow on your tent it will sag because it is heavy. I don't own any true 4 season tents, but I have been in snow and that requires you to knock it off the tent roof every now and then.
Tents differ in how warm they are, depending if they are single skin or dual skin and how much mesh they have. But 3 or 4 season doesn't refer to that. Most tents are dual skin, but my most used tent is single skin (or 1.5 skin - Tarptent Double Rainbow) and that one is very cold, also because it needs lots of mesh and ventilation to reduce condensation. I have never measured the actual temperature difference, but my dual skin tents feel much warmer.
Mattress is very important, because you lose lots of heat through there. This becomes even more important with better quality sleeping bags. This is because cheap sleeping bags are not very compressible, so you still have some insulation underneath. But with the lighter, highly compressible sleeping bags, you have virtually nothing underneath you except the mattress.
I have two mattresses: NeoAir XLite (R=3.2) and NeoAir XTherm (R=5.7). With the first one I have camped on frozen ground at minimum temperatures of about -3°C. For myself, that was really the limit for that mattress and I wished for a warmer one at the time, even though I didn't get uncomfortably cold yet. For that reason I got the XTherm afterwards. I really like it, now I never get cold from underneath. However, in most cases it would be overkill. I still use both matresses. Because the Xlite packs smaller and is lighter, I will bring it if it doesn't get colder than -1°C (the majority of my hikes). Below that I go with the XTherm (I like to have the extra margin). If I were to buy only one right now, it would be the XTherm, because it also performs great at higher temperatures. In fact, it also does a great job insulation from hot ground that had been exposed to the sun during the day (think deserts etc), which can make uncomfortable sleeping at the beginning of the night.
As for sleeping bags, I would say buy the best you can afford. It is really you get what you pay for (good quality down is expensive). The best down bags (high fill power) are very compressible and light, and they last for ages (make sure you store them uncompressed!). Adding up all trips I've made with my current bag, I estimate that I have slept a total of more than 14 months in it, which also includes stuffing/ highly compressing it all these days, which I presume puts stress on the down. It still has great loft and I expect it to last for many more years. In my case it has outlasted most other gear, so it is worth spending money on.