Okay, it's viotech3 here, but I'm here to report that C4 jumping is still doable, and it's 100% reproducable.
Put 1 or 4 C4 down. you *can* do this with 2, but it's fiddly. 3 C4 kills you 99.9% of the time. 4 is likewise gimmicky. 1 is safe 100% of the time.
So, normally c4 kills you when you jump. However, if you ragdoll+explode the c4 at the same time, you get launched and take no damage. This means it's easily reproducible. However, note, this might not be easy to fix without screwing over ragdolls. As of right now, Lopez discovered it by accident, and he and I perfected it while fiddling on early Constructionside. We've both agreed not to use in actual gameplay and to not tell others. Very few people know about it and only me and lopez can reproduce it accurately.
I'll leave the solution to you, just wanted to make sure you knew.