Just replaced my vizio 4kUHD LED main digital board, but am still having horizontal lines causing screen distortion.

This sounds like a panel issue unfortunately, if its under warranty or you purchased an extended warranty great, if not your kind of SOL

It could be a defective main power supply, or a defective main digital board. I will try a new main board first.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electro-medical.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Line Defects On Vizio TV Screens

Could be a bad CCFL lamp, if the set usess CCFL back-light system. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electronicshelponline.blogspot.com/ To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Fault at its main digital board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electronicshelponline.blogspot.com/
In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.
Before going further; try a hard reset first. If it does not help you, replace the main digital board. There are two methods to reset TVs. The first is simple to do; a Hard Reset. The other is somewhat skilled; is a Factory Reset (System Reset). To make it a hard reset, just unplug the TV from AC mains wall socket, and re-plug it back after 15 minutes or so. If you do this by overnight is better. This procedure will make it a hard reset. If you want to make it a system reset [Factory Reset]; you have to enter its service mode option, and select the factory reset option. If you wish to get some details; visit the site linked here. ...

Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
Some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement.

In what way is the screen broken? cracked, distorted picture, no picture but has sound?

If it is cracked. Replacement of screen is expensive. Cheaper to buy a new TV.
If Distorted. Parts linked to the screen can be the culprit. Most common part to fail is the T-con board. Main board may not be sending signal to the T-con board.
If its no picture but has sound. Back lighting has failed which could be several things that can cause that.

without any other details besides "broken" it will be hard for someone to help with diagnosing your problem. We need symptoms like. has sound but no picture. Picture has horizontal or vertical lines. part or half of the screen is black, black borders don't count. Hope the info here has, in some way, helped bring you closer to what to do with your TV.

It's not the motherboard you have to replace it's the resistors board, it's an LG board 038A to an upgraded 038C or 060Q. Told the 060Q is the best.

Hi There are three things that can cause this problem. Your lcd screen, t-con board or the signal board. Most likelly in your case since you say it's only half the screen it's going to be your t-con board. You can try removing it and cleaning all the contacts and replacing it see if that helps. If that doesn't help then replace it. If that does not solve it then it's going to be your screen.

You can get parts at www.interliquidparts.com