Many of us on here ride our bikes on track to a lesser or greater extent so its not unusual to find that at all and the bike is designed to do it, so a bike thats done a few track days is absolutely fine, if its been used as a dedicated track bike that's a bit different and you'd need to proceed with more caution. As always you have to be critical of a secondhand bikes condition, service history, type of owner if its a private purchase, HPI check etc. Some very smart HP4's around and they hold their value very well, take your time finding a good one. You'll get plenty of advice on here and maybe even the offer of a bike or whereabouts of a decent one.