I *think* the best deal is to use Jampoints to buy enough gear credits to purchase Amp 4. The gain here is that the number of gear credits you get is higher than the amount you have to spend to get them (and Jampoints gets you 30% off the gear credits price).
So, you can get 87 gear credits by spending $52.50 (where you've reduced the price by 30% using Jam points). If you had 13 credits in your account already before that point, then you will have purchased Amp 4 for $52.50.
Otherwise, you have to spend $70 either to buy it directly with Jampoints or to purchase a 120 gear credit pack. In that latter scenario, you get 20 extra gear credits in the process so you're still better to purchase the gear credits.