It is not possible to build media-video/ffmpeg-4, since media-plugins/gst-plugins-libav-1.14.2 block.

Looking in gst-plugins-libav-1.14.2.ebuild reviles:

From a reason i do not know, libav is masked and that is why it is not possible to select libav use flag

emerge -1va gst-plugins-libav:

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild     UD ] media-video/ffmpeg-3.4.5:0/55.57.57::media-kit [4.1:0/56.58.58::media-kit] USE="X alsa bluray bzip2 cdio chromium encode gpl hardcoded-tables iconv ieee1394 jpeg2k libass mp3 network opengl openssl postproc pulseaudio sdl
svg theora threads truetype twolame v4l vaapi vdpau vorbis vpx wavpack webp x264 x265 xvid zlib (-altivec) -amr -amrenc -appkit -bs2b -chromaprint -cpudetection -debug -doc -fdk -flite -fontconfig -frei0r -fribidi -gcrypt -gme -gmp -gnutls
-gsm -iec61883 -jack -kvazaar -ladspa -libcaca -libdrm -libilbc -librtmp -libsoxr -libv4l -lzma (-mipsdspr1) (-mipsdspr2) (-mipsfpu) (-mmal) -modplug -nvenc% -openal -opencl -openh264 -opus -oss -pic -rubberband -samba -snappy -speex -ssh
-static-libs -test -xcb -zeromq -zimg -zvbi (-codec2%) (-libaom%) (-libressl%) (-lv2%)" CPU_FLAGS_X86="3dnow 3dnowext aes avx avx2 fma3 fma4 mmx mmxext sse sse2 sse3 sse4_1 sse4_2 ssse3 -xop" FFTOOLS="aviocat cws2fws ffescape ffeval ffhas
h fourcc2pixfmt graph2dot ismindex pktdumper qt-faststart sidxindex trasher" VIDEO_CARDS="(-nvidia%*)" 0 KiB                                                                                                                                    
[ebuild  N     ] media-plugins/gst-plugins-libav-1.14.2:1.0::media-kit  USE="orc (-libav)" 8,825 KiB

Total: 2 packages (1 downgrade, 1 new), Size of downloads: 8,825 KiB

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

media-video/ffmpeg:0

 (media-video/ffmpeg-3.4.5:0/55.57.57::media-kit, ebuild scheduled for merge) pulled in by
   <media-video/ffmpeg-4 required by (media-plugins/gst-plugins-libav-1.14.2:1.0/1.0::media-kit, ebuild scheduled for merge)
   ^                   ^                                                                                                                                                            

 (media-video/ffmpeg-4.1:0/56.58.58::media-kit, installed) pulled in by
   media-video/ffmpeg:0/56.58.58= required by (kde-apps/k3b-18.08.3:5/5::kde-kit, installed)
                     ^^^^^^^^^^^^                                                                                                
   (and 7 more with the same problem)

NOTE: Use the '--verbose-conflicts' option to display parents omitted above