Yes, and yes, and... they don't need mix oil like a 2 stroke. I've seen more han one Stihl 4-mix blower that has had a tank of straight gas, and they survived with little damage (don't try a second tank though...). Oil accumulates in the crankcase. If you use an excessively rich synthetic mix you end up with a sticky mess running from your muffler, excessive oily junk in the crankcase pan, and "odd-ball problems". I can't talk from experience with C4- you need to seek assistance from Shindawa - call them.
In your 4-mix engine, run 50:1 synthetic unless your manf. tells you you can run less oil. Stihl allows us to tell 4-mix blower users they can run 80:1 on Ultra, but we don't bother as its too confusing to have 2 ratios of gas and we doen't see any problems with 50:1 Ultra.
Even Ultra at 50:1 will leave a sooty residue in the exhaust , but it does not seem to affect the valves.