Re: 90 YJ Engine
I've noticed you asking a lot of questions about different years, so I hope this helps the search for the "perfect" Jeep.
Yes it is. The 4.0 came out for the '91 model year.
87 - 95: 2.5l inline 4 w/ fuel injection
87 - 90: 4.2l inline 6 w/ BBD carberator
91 - 95: 4.0l inline 6 w/ fuel injection
87 - 91: old style roll bar w/45* down bars
92 - 95: new style roll bar w/full hoop rear bars
Not many other notable differences other than the larger knuckle U-joints on the front axle in '95 and the '87 had an NP208 transfer case and all others had an NP231.
The side mirrors were different in 87-88 and for these two years, the Peugot 5-speed tranny was used which doesn't have a very good rep
'88 Sahara, 4.2, auto, 4.0 head swap (finally), no trac bars.