The 1837 edition introduced the present-tense is in place of the original past-tense was. This change was not marked by Joseph Smith in the printer’s manuscript, so one might question whether the 1837 change to is was actually intended. The past-tense was also agrees with the past tense in the main clause (“it was accounted”).
The use of the past tense suggests the possibility that Abraham himself realized (even if only afterwards) the symbolism in the sacrifice of his son Isaac. Of course, any act of symbolism in the past can remain symbolic in the present, so the switch to the present tense is not really necessary.
It is also possible to interpret the past-tense was in the relative clause as a case of tense backshifting, where the was in the relative clause “which was a similitude of God and his Only Begotten Son” occurs because the preceding clause is in the past tense (“even as it was accounted unto Abraham in the wilderness”). A striking instance of such usage is found later in Jacob 7:22 (“which was in heaven); this example of was has never been replaced by the present-tense is:
The whole passage in Jacob 7:22 is in the past tense (“was pleasing ... had requested ... had heard”), including the relative clause “which was in heaven”. Of course, we expect “my Father which is in heaven”. For further discussion of the possibility of tense backshifting in relative clauses, see the discussion under Jacob 7:22.
Summary: Restore in Jacob 4:5 the past-tense was, the reading in the printer’s manuscript (the original manuscript is not extant here).