My understanding is that, in UEFA football games, a forfeiting team is awarded a 0-3 score (defeat) against the opposing team. If, in a competition, a team wins the first game 4-0, could the same team decide to forfeit the second leg, so losing 3-0 and assuring it would still qualify to the next round? (I understand this is hypothetical)
Since you explicitly mentioned the UEFA, I'll answer your question using their regulations. And the answer is no. That's not possible.
The UEFA does not allow this behavior in their competitions.
If a team does not play a match or is responsible for it to not take place in any way that team will be disqualified and fined. This means the opponent will advance to the next round, if applicable.
Refusal to play and similar cases
If a club refuses to play or is responsible for a match not taking place or not being played in full, the UEFA Control, Ethics and Disciplinary Body declares the match forfeited and disqualifies the club concerned in combination with the following fines (...)