Whoever tells you its the bearings, i'll tell you now will be banned!!!
Synchros, selector ring or dogteeth (or slight wear on both which, together causes problems) 4th and 5th are on the same selector clustor.
Might also be the 3 ball bearings in the top (under the small steel plate) these can get clogged with shit and can make shifting feel like crap... or non-existant!
Has your box ever been rebuilt? When Jap T50's are rebuilt, generally only 1st, 2nd and 3rd are changed as 4th and 5th are rarely buggered, after a couple of swaps though, they do catch up.
Might also be your master or slave, but you would have problems with selecting all gears.