It looks like my 4000. That's the only model with chromed lugs, ever. Andrew Muri at yellow jersey can confirm. Here's a web page including the 4000 :
1977 sekai flyer catalogue, unfortunately missing a "chunk" :
http://www.ece.ubc.ca/
You can check the dropouts to confirm that they are the correct types. The fork is actually a tange professional - not made by SEKAI. Your decals are post-1977, the 1977 decals would say "SEKAI" and then "4000 Professional" in smaller script, stacked, right next to "SEKAI". the tange champion decals were, roughly :
green - pre 1977
http://www.ece.ubc.ca/
- Don Gillies
San Diego, CA