Not so important, but there is a check for 4096x4096 res limit in the encoder, so I belive that this should work.
C:\>ffmpeg -i 4000.jpg -vcodec flashsv out.flv
ffmpeg version N-66438-g4f4f08e Copyright (c) 2000-2014 the FFmpeg developers
built on Sep 24 2014 22:26:49 with gcc 4.9.1 (GCC)
configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-lib
modplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrw
b --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinge
r --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --en
able-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis
--enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-
libx265 --enable-libxavs --enable-libxvid --enable-zlib
libavutil 54. 7.101 / 54. 7.101
libavcodec 56. 1.101 / 56. 1.101
libavformat 56. 5.100 / 56. 5.100
libavdevice 56. 1.100 / 56. 1.100
libavfilter 5. 1.102 / 5. 1.102
libswscale 3. 1.100 / 3. 1.100
libswresample 1. 1.100 / 1. 1.100
libpostproc 53. 1.100 / 53. 1.100
Input #0, image2, from '4000.jpg':
Duration: 00:00:00.04, start: 0.000000, bitrate: 268008 kb/s
Stream #0:0: Video: mjpeg, yuvj420p(pc, bt470bg), 4000x4000 [SAR 144:144 DAR
1:1], 25 tbr, 25 tbn, 25 tbc
[swscaler @ 02f27000] deprecated pixel format used, make sure you did set range
correctly
Output #0, flv, to 'out.flv':
Metadata:
encoder : Lavf56.5.100
Stream #0:0: Video: flashsv ([3][0][0][0] / 0x0003), bgr24, 4000x4000 [SAR 1
:1 DAR 1:1], q=2-31, 200 kb/s, 25 fps, 1k tbn, 25 tbc
Metadata:
encoder : Lavc56.1.101 flashsv
Stream mapping:
Stream #0:0 -> #0:0 (mjpeg (native) -> flashsv (native))
Press [q] to stop, [?] for help
frame= 1 fps=0.4 q=0.0 size= 17693kB time=00:00:00.04 bitrate=3623582.0kbit
frame= 1 fps=0.4 q=0.0 Lsize= 17693kB time=00:00:00.04 bitrate=3623582.0kbi
ts/s
video:17693kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing
overhead: 0.001253%
C:\>ffmpeg -i out.flv out.avi
ffmpeg version N-66438-g4f4f08e Copyright (c) 2000-2014 the FFmpeg developers
built on Sep 24 2014 22:26:49 with gcc 4.9.1 (GCC)
configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-lib
modplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrw
b --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinge
r --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --en
able-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis
--enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-
libx265 --enable-libxavs --enable-libxvid --enable-zlib
libavutil 54. 7.101 / 54. 7.101
libavcodec 56. 1.101 / 56. 1.101
libavformat 56. 5.100 / 56. 5.100
libavdevice 56. 1.100 / 56. 1.100
libavfilter 5. 1.102 / 5. 1.102
libswscale 3. 1.100 / 3. 1.100
libswresample 1. 1.100 / 1. 1.100
libpostproc 53. 1.100 / 53. 1.100
Input #0, flv, from 'out.flv':
Metadata:
encoder : Lavf56.5.100
Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
Stream #0:0: Video: flashsv, bgr24, 4000x4000, 200 kb/s, 1k tbr, 1k tbn, 1k
tbc
Output #0, avi, to 'out.avi':
Metadata:
ISFT : Lavf56.5.100
Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 4000x4000, q=2-31, 2
00 kb/s, 1k fps, 1k tbn, 1k tbc
Metadata:
encoder : Lavc56.1.101 mpeg4
Stream mapping:
Stream #0:0 -> #0:0 (flashsv (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
Error while decoding stream #0:0: Invalid data found when processing input
frame= 0 fps=0.0 q=0.0 Lsize= 6kB time=00:00:00.00 bitrate=N/A
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing ove
rhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters i
f used)
Conversion failed!
This is "fixed" by f0158e6f0cc1bd6bf5d0485a325bdc654fcd3328 by saste
4000x4000 works fine if the encoded frame fits within the limits of flv