4 may go to the police in regard to 1, the alleged thief. Apparently he is reluctant to do that for some reason, which is not our business. It would be wrong to go to the police concerning 3, who is a victim, not a culprit, even though we have yet to determine if 3 reacted correctly or not. As we always point out in cases like this, we cannot make any determinations to obligate someone who has not had the opportunity to present his side formally before beit din. We can only tell someone how to act, according to his version of the story, until there is an agreement between the sides or a din Torah.
When an object is stolen, it is normally incumbent on the thief to return it.[1] For that matter, anyone who has access to the object has a mitzva to return it as a lost item. However, situations may occur that remove the original owner’s control over the object. One factor is yei’ush, when the original owner gives up real hope of recovering the object. If there was yei’ush and the thief subsequently sold it to someone else, the buyer acquires the object and does not need to return it to the original owner.[2] If that is the case here, then 3 can either keep the computer or offer it sell it back to 4 at whatever fair price he likes.
There are rules that help determine whether yei’ush can be presumed. The general rule is that when one Jew steals from another Jew, whether forcefully or unobtrusively, there is a presumption of yei’ush.[3] However, the Rama contends that the proper custom is to return the stolen article to its original owner in any case, a practice that apparently began because of the law of the land.[4]
From the sound of your description, it seems likely that there were special circumstances under which there might not have been yei’ush. Let us investigate the halacha in that case. When a third party buys a stolen object from the thief and there was no yei’ush, basic legal principles dictate that the buyer has wasted his money, and the victim receives the object back without having to reimburse the buyer. Nevertheless, there is a rabbinical provision (takanat hashuk) to reimburse the buyer for the amount that he spent on the object. This was instituted out of concern that people would be overly nervous that they would lose money by buying stolen objects.[5] However, the Rabbis realized that this is fair only if the buyer did not have reason to believe he was buying a stolen object. However, if the seller was a known thief[6] or at least if the buyer knew it was a stolen object,[7] he does not deserve reimbursement.
In this case, the buyer knew he was buying a stolen object and so it differs from the standard case for which the takanat hashuk was instituted. On the other hand, perhaps the takanat hashuk was a broad one, applying to any case where the buyer cannot be accused of foul play, of knowingly or semi-knowingly buying a stolen object for his own benefit. Your depiction implies that the buyer’s intention seems to have been reasonable. From the general approach of the poskim, it appears that the more inclusive outlook on the takana is correct. Thus, we believe (bearing the caveat above in mind) that 3 has a right to demand the 1,500 shekels from 4, whether or not the latter had experienced yei’ush.