Concatenate four MP4 video files using FFmpeg - ffmpeg

Question

I'm trying to concatenate four MP4 video files using FFmpeg. (latest version) Those four mp4 videos differ from general mp4 videos and have specific codecs, I play them in Windows Media Player with the help of K-Lite Mega Codec Pack 13.1.0.
Codec information of one file (all four files have same codecs) by MediaInfo:
General
Complete name : Testing - 1.mp4
Format : MPEG-4
Format profile : Base Media
Codec ID : isom (mp41/avc1)
File size : 59.4 MiB
Duration : 3 min 47 s
Overall bit rate mode : Variable
Overall bit rate : 2 196 kb/s
Writing application : vlc 2.2.4 stream output
Comment : QuickTime 6.0 or greater
Video
ID : 2
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High#L3
Format settings, CABAC : Yes
Format settings, ReFrames : 3 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 3 min 47 s
Bit rate mode : Constant
Bit rate : 2 000 kb/s
Width : 720 pixels
Height : 576 pixels
Display aspect ratio : 5:4
Frame rate mode : Constant
Frame rate : 25.000 FPS
Standard : PAL
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.193
Stream size : 54.1 MiB (91%)
Audio
ID : 1
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 3 min 47 s
Source duration : 3 min 46 s
Bit rate mode : Variable
Bit rate : 192 kb/s
Maximum bit rate : 294 kb/s
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 kHz
Frame rate : 46.875 FPS (1024 spf)
Compression mode : Lossy
Stream size : 5.13 MiB (9%)
Source stream size : 5.13 MiB (9%)
MP4 video files to be concatenated are "Testing - 1.mp4", "Testing - 2.mp4", "Testing - 3.mp4" and "Testing - 4.mp4".
I needed to use FFmpeg's Concatenation method to join those four MP4 files together into one MP4 file.
First attempt (using Concat Demuxer):
FFmpeg -f concat -safe 0 -i "F:\New Text Document.txt" -c copy E:\Testing.mp4
File concatenated into one MP4 file with several errors and warnings from FFmpeg.
Non-monotonous DTS in output stream
SPS NALU is missing or invalid, The resulting stream may not play
When playing above concatenated file with Windows Media Player, it gives error
Network is too busy to play file at original quality
but the file actually in a local hard drive.
Seek option not working (even in VLC media player) and Windows Explorer showing incorrect duration like 3h 54min, it should be 15min 05s.
Second attempt (using Concat Demuxer):
FFmpeg -f concat -safe 0 -i "F:\New Text Document.txt" -vcodec copy -acodec copy -crf 0 E:\Testing.mp4
Then file concatenates successfully and plays without any errors in Windows Media Player and VLC media player. The only problem here is file is very big and about 1.2GB in size because Constant Rate Factor is set to 0 to stop WMP network related error.
Note: Before concatenating (four source files) are playing fine in both players without any errors, but they are medium in size also. As MediaInfo shows, "Testing - 1.mp4" is 59.4 MB, instead huge sizes.
When I join those using EasiestSoft Video/Audio Joiner 4.7, file produces no errors while playing (I mean everything works fine as expect, but I need a command line tool like FFmpeg to batch process multiple videos), but in version 5.0, it seems to produce this network related error (this may be a program bug, I think it re-encodes even I check don't re-encode checkbox).
What can I do with FFmpeg to make this concatenated MP4 file smaller (around total size of four source files) and to make this able to play in both players without any network related issues or incorrect duration issues.