4 meter is a little short for a 200MHz scope. The fault could be hidden in the rising edge.
I have a couple techniques, depending on the situation, when the TDR route is impractical.
My most favorite gadget is a Carlson Superprobe (diy project from youtube). Instead of using a piece of coax as the probe, I added an SMA connector so I can change from an E-field probe (for open circuits) to an H-field probe (for short circuits). Audio is a bit tricky getting any distance with, but if you AM modulate a LF carrier, you can easily reach through a wall. Just don't go too high in frequency or it can couple to other conductors.
Another one of my tricks for really getting through walls, and this is primarily for following good cables, not necessarily for finding breaks, is to inject an HF carrier at the farthest outlet between the ground and neutral using a simple crystal oscillator and decoupling cap connected to an extension cord end and plugged in, and using a handheld radio scanner as a receiver with an H-field probe in place of the antenna. In this case, since the detection is with the H-field, current flow is required. Being the ground and neutral are connected in the box, that requirement is met. Assuming your break is in the hot wire, this may still work in your case for following the path of the wire.
Edit: I mention the latter HF + H-field trick because knowing where the cable is in the wall greatly aids in finding the weak e-field signals on the broken wire. As has been said, getting a tone from a few inches away is sometimes tricky, but knowing right where to look may help you pick up the signal after losing it in a corner or something.
Edit2: and don't be afraid of trying to get a tone on the outside of the wall. In some pre-fab or lazy builds, instead of drilling a hold in the stud, a notch is cut into the face of it and covered with a metal plate. Often times, these are on the outside face of the wall,so getting a tone on the inside may be hard.
I built one of those Mr.C Superprobes, and have used it for checking wiring. But yeah it has limited range through walls. That was using ~1.2kHz test signal, I never tried RF. So how much better is it with the E and H field sensors? What kind were they ?
I have a string of Christmas lights that doesn't light up any bulbs, so I'm curious to see what my Superprobe would find. They might have had a fuse, or the 1st bulb is gone I guess.
I will set up a little demonstration and put it on youtube. My H-field probe is just a simple homemade one. Basically, a 1" loop from the center of the SMA to the ground side (cant remember if I did one turn or 4-5 turns, but that hardly matters), and then a grounded shield around that with a split in the shielding so it isn't electrically continuous around the loop. That blocks the electric field but not the magnetic field.
Give me a few hours and I will make a video showing the difference between audio and modulated LF on the carlson superprobe, and the usefulness of an H-field probe attachment.
I should also add for the OP (since he was in a panic about RF) that coupling to other wires can be mitigated to a workable degree with careful choice of frequency. Take underground wire tracing for instance. Where I live, it is usually done in the kHz, not 60Hz. The frequency choice is a tradeoff between the distance you need to go (how long the wire is), the depth of the wire (transmission range needed) and the amount of coupling to the other wires that you can tolerate. Lower frequencies go farther if the wire is long and couple less into adjacent wires if they are present and parallel to the driven conductor, but the transmission range takes a hit because the wire is nowhere near the length of a proper radiator. Since his situation is a short run, coupling would be insignificant under probably about 30kHz, and given the short run, there would be no problem making it to the end at that frequency. In other words, wire tracing should be done at the highest frequency possible that is unlikely to couple into adjacent wires that can still make it to the other end and transmit through the thickness needed. If you try a frequency and the coupling is so great that you cannot tell from the drop in signal that you are on another conductor, drop the frequency a little. Practice practice practice, don't just assume all RF must be avoided.