capacitive divider?
Resistive divider! Nah, not efficent, just a drop out resistor!
Ha... 4kW @220V, so you need 18A, 3.9Ω resistor of barely 1260W, then you just connect your load (a bridge rectifier in between). Put a zener, so if the load is unconnected you still get 50V, that's a 900W zener, buy one in the drugstore!
Now, seriously, what kind of what are you trying to power up... SCRs are like triacs for DC, or more precisely, to convert from DC to AC but with the kind of waveform triacs do. Power supplies are either linear (with transformer) or switching.
You could use something like what you described to make what I think you want, in any case you could need some kind of storage (cap) if you actually need DC with any reasonable ripple. Now you can trigger the SCR at the point, about 3/8 wave after the zero crossing so the output has ~50V peak and you charge that cap for your load to use. Is that what you expect?
JS
Edit: one thing you might not notice, a half wave rectifier DC is 0.45 times the input voltage, so with a 36A diode you can have your 50V DC you want, it will be a half wave rectified signal, then if you want to get a steady DC and not just some average of a crappy thing, just a cap will make the voltage way too high, so you will need some inductance as well.