my 93 disco did something similar but I couldnt get 4th at all, on pulling the top apart as prep to get the box out saw the busted tab sitting there on the selector ball at the back mechanically blocking out 4th.. pulled it and welded it and all went well till the guide spring broke then it was pretty much a lotto as to weather you got the 3/4 selector or the r/5 fixed it when I sold it as part of the sale deal.
IF that aint it then your syncro spring or clutch cone is U/Sish on fourth, changing up the gears will mean your trying to speed up the ass end of the box changing down means that they are slowing down and will help whats left of the syncro do its job. if your any good at it give double cluthcing a go and see if you can get it in.
