I updated my Vero4k+ for the first time last week. It’s the first time since then I’ve played any 4:3 content, these are MakeMKV ripped DVDs and I’ve never had an issue before with any player.
Playing back some Inspector Morse rips and they’re playing back as 16:9? The info is saying they’re 1.78:1 as well.
Is it worth clearing the library first and trying again?
My TV has a 4:3 mode but that cuts some bits of the video off and applies across the input rather than just the Vero4k+ since it’s coming through the receiver.
Are you running v17 Krypton or v18 Leia?
v17 Krypton, here’s the MediaInfo of an example file…
General
Unique ID : 91263150914293889976907907984283273488 (0x44A8A4BD7F4507117BD26935B7D20910)
Complete name : U:\tv series\inspector morse\season 05\inspector morse s05e01.mkv
Format : Matroska
Format version : Version 2
File size : 3.94 GiB
Duration : 1 h 43 min
Overall bit rate mode : Variable
Overall bit rate : 5 434 kb/s
Encoded date : UTC 2010-04-28 17:05:11
Writing application : MakeMKV v1.5.2 beta win(x64-release)
Writing library : libmakemkv v1.5.2 beta (0.7.7/0.8.1) win(x64-release)
Video
ID : 1
Format : MPEG Video
Format version : Version 2
Format profile : Main@Main
Format settings : CustomMatrix / BVOP
Format settings, BVOP : Yes
Format settings, Matrix : Custom
Format settings, GOP : M=3, N=12
Codec ID : V_MPEG2
Codec ID/Info : MPEG 1 or 2 Video
Duration : 1 h 43 min
Bit rate mode : Variable
Bit rate : 5 133 kb/s
Width : 720 pixels
Height : 576 pixels
Display aspect ratio : 16:9
Original display aspect ratio : 4:3
Frame rate mode : Constant
Frame rate : 25.000 FPS
Standard : PAL
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Compression mode : Lossy
Bits/(Pixel*Frame) : 0.495
Time code of first frame : 00:00:00:00
Time code source : Group of pictures header
Stream size : 3.72 GiB (94%)
Language : English
Default : Yes
Forced : No
Audio
ID : 2
Format : AC-3
Format/Info : Audio Coding 3
Commercial name : Dolby Digital
Codec ID : A_AC3
Duration : 1 h 43 min
Bit rate mode : Constant
Bit rate : 192 kb/s
Channel(s) : 2 channels
Channel layout : L R
Sampling rate : 48.0 kHz
Frame rate : 31.250 FPS (1536 SPF)
Bit depth : 16 bits
Compression mode : Lossy
Delay relative to video : 1 s 85 ms
Stream size : 143 MiB (4%)
Title : 2/0
Language : English
Service kind : Complete Main
Default : Yes
Forced : No
Menu
00:00:00.000 : en:Chapter 01
00:12:38.480 : en:Chapter 02
00:27:41.200 : en:Chapter 03
00:40:28.400 : en:Chapter 04
00:55:14.360 : en:Chapter 05
01:07:22.520 : en:Chapter 06
01:20:19.840 : en:Chapter 07
01:33:30.200 : en:Chapter 08
I just noticed Display Aspect Ratio is 16:9 
OK, probably not an issue, I assume they changed soomething in MakeMKV between rips. An earlier rip has Display Aspect Ratio is 4:3 and no Original display aspect ratio.
Yep, fixed that and it works. I guess MakeMKV has been playing silly buggers when versions updated.