But not for an excellent performance from Lions keeper Lewis Boughton, the Hornets could have added considerably more to the scoreline.
The hosts made five changes from Saturday's FA Cup first round clash at League Two side Carlisle United.
New signing Leon Moore, Danny Dudley, Tom Richards, Lucas Rodrigues and Shamir Fenelon replaced Steve Metcalf, Will Miles, Harvey Sparks, Tom Kavanagh and Lee Harding.
Horsham, having had a shot cleared off the line a minute before, took the lead on 12 minutes. The Lions failed to clear their lines from a corner and Fenelon was on hand to fire home from close range.
The Hornets added a second six minutes before the break. Dudley was left unmarked and the defender converted a cross at the far post.
Horsham extended their advantage 11 minutes into the second half. A deflected shot landed at the feet of Charlie Hester-Cook and the midfielder duly netted his first Hornets goal.
And the hosts added their fourth on 78 minutes. Brivio headed home the rebound after a good stop from Boughton.
Di Paola said: "I thought the attitude of the boys was brilliant. We didn’t switch off from the weekend, which was quite draining. A lot of the boys got back quite late on Sunday evening.
"I thought it was a really professional performance. They only really had a couple of shots, and one a half-chance, but the rest was all us.
"There could have been a lot more goals to be honest but there were lots of positives.
"We got an hour into Shamir and a few of the boys that were on the bench on Saturday that got starts.
"It was all positive really. It was a good performance."