people throw 300 watts + to the hsk series regularly Also just because its 280 watts doesnt mean you need to use all 280 watts. It doesnt work that way the actual amount of watts your speakers see depends on a large variety of factors and more often then not, you'll never get close to the amount of wattage on the amp's sticker due to how music works.
More often then not you have people with not enough power turning it up/overdriving the amp and getting a dirty signal which leads to distortion, clipping and blown speakers.
Now, the reason why you'd want a 4 channel is because you want to eventually ditch the passive crossover on the hsk set and run active on that pioneer head unit you have. Why? Because passive crossover is a set crossover point, it might not work well with your car's acoustics and it limits actual power the speakers see.
When going active network the tweeter and mid are plugged straight to the amp's channels and all the crossovers are done on the head unit to get the perfect smooth blend between mid and tweeter and they both get ample amounts of wattage. This may sound over your head but its how people achieve sound quality magic rather than shoving a pair of components in the doors and praying to god the passive crossover does the job.