The 29-year-old touched down twice after James Davies scored the Scarlets' first try, and Steve Shingler added ten points with his boot.
The Welsh region opened up a 15-point gap at half-time while holding Leinster to nil, but to their credit Leo Cullen's men came back into the contest before it finished 25-14 in Scarlets' favour.
It perhaps hasn't been quite the perfect start - they've not been able to pick up any four-try bonus points in their four wins - and Pivac says his team are intent on keeping their feet on the ground.
"At the start of the season, if you had said we would be four wins from four we would be very happy, but we still haven't put together an 80-minute performance," he said.
"The boys are aware of that and that is why there are not too many celebrations going on in the dressing room. We will go away and try and improve for Munster.
"In the first half we blew a few opportunities, the turnover rate was 40 percent of our ball, six handling errors and six turnovers in the contact.
"We spoke about that at half-time. I thought we started the second half well, we dominated possession and territory, managed to cross the line and pretty much secure the win. Then it was about trying to get the bonus point.
"You have got to give some credit to the defence shown by Leinster, they have lost a lot of players and the players that showed up really got stuck in and put us under some pressure.
"We just needed one or two of those passes to stick and it may have been a different ball game, but it was satisfying to get another four points."
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