"I see that the 1.8.0 Upgrade is released in 4 parts... does this mean I will have to pay 4 times for each part of the upgrade?"

No.  When you purchase the 1.8.0 upgrade you will get all 4 segments of the release. 

The reason we are doing this is because the release is so large that it nearly doubles the features of our existing kart and in order to safely & effectively compile, install & deliver this release we want to be able to thoroughly test each segment before moving on.