I have 4 spiny cacti in one flowerpot and after few years one of them is starting to grow more side-wards and getting closer to the other cacti (spines are already touching). I'm just wondering if it's going to negatively affect how they grow like stabbing each other and causing damage or if they won't grow further into each other and will co-exist? Should I re-pot them?
No, not really. You don't have them planted too dense either, so they don't need re-potting. A different story would be if you'd have them packed really dense, because that causes them to stretch (and thin out) at the bottom (whilst the top remains thick), making for an unsightly mess and crooked cacti. Make sure they receive a lot of sun though, otherwise they will become overly long and thin.