The servo's aren't great... Just replace them. I know it will cost nearly $50, but it's worth it.
I tried HS-55 and they were OK for a while, but are too sensitive to noise for the WK4.
I got some cheap ($22) "Manufactured in China" PFS-09 "Park Flyer R/C Servo's", which have been great. I originally bought them for the tail servo for my Walkera 22a (which flew for the second time today - last flight before I replace the tail gears...). They are good. Very resistant to noise.
Unlike all other servo's I've tried, they don't budge. If you turn the receiver on and off, they remain steady... Not a sound. And since you have center stick when you turn off the transmitter/receiver, they don't move when you turn on either. I literally have to swivel the stick to see them move.
They are a little slow, but fast enough for a WK4. And now I am getting some nice hover... Just dealing with the "Power Cut" problem now... I'm starting to wonder if I have a bad battery connector or maybe a broken aerial wire. Some time I will replace the copper wire with a multicore.
I've gotten mine to take off straight up to a stable hover now. Just add power... But like most Walkera 4's it needs constant attention.
Still I've flown it inside for the first time... Not bad with just 50cm clearance in four directions. Most flights end in grief, but I've kept it close to
center for a 10 to 12 seconds. You need about 1m to 2m clearance in each direction to fly it inside well.
I went through 10 tail rotors. I always buy one more than I have fitted. (Always keep a spare). The original had longer legs than the cheap replacement tailrotors I get from modelshops. I originally put a wire tail on, fixed in 2 spots, but it eventually got bent out of shape. I only went back to the stick today. I got some 1.2mm wire, folder the end 3mm, and stuck it in the hole next to the motor. Then I bent the end around in a half-circle and hooked the end around the tailstick. It keeps it from moving into the tail-rotor arc. A stick of 1m long 2mm carbon stick is only $8, so get some. You will need it. Make the tail stick a centimeter longer. And run a diagonal wire down the line of the tail flag (which I got rid of anyway) and make the end "Bouncy"
That slowed down the destruction of tail rotors a lot for me! Also, I've thought of gluing the broken hub center from a tail rotor to the bottom of the tailstick.
The Walkera 4 isn't too fussy about CoG. They are close anyway. Moving the battery isn't hard, but a few grams at the tail doesn't matter.
My Main rotors are badly chewed out now from impacts... I'm even starting to wonder if that's causing the Gyro to glitch, but the thing still flies, and
when it's not glitching, it's quite good to fly.
The biggest thing with the Walkera is to fix up the head. Clean up any extra plastic.
Also, you usually need to make the blades tight, and they work much better with a 8cell battery
Much longer flying times.
Anyway, my experiences with the Walkera 4 have given me an idea what is needed in a training heli
Maybe one day I'll design some simple changes and suggest them to the factories