Tweaking the 4's - Do not underestimate

Hi people, I've already did one post and was wondering about the 6 cyl swap, that is still always a possibility, but the fours cant be underestimated. I just got home from driving with my dad, after he did some tweaking and fixed the fuel injectors, the car is very different. Honestly. I asked him what did he do, but he just said he tweaked it, and I am pretty dam sure i hit 60 mph in less than 8 seconds maybe even faster, but I dont want to over dramatize anything, I was doing 75 before I knew it. But hey, I cant judge time very well, because Im sure it was more. The funny thing about that is we were next to a Firebird or something from Pontiac, and at the next light , he thought I was trying to race him, but I was just testing the car. Anyway, does "tweaking" the engine really do that much??? I've heard Skylines can do 320 hp stock or even more with tweaking of the engine only. I'm sure that it wasnt the same kind of tweaking on my car, but it really felt different. Although I still want a 6cyl., I dont doubt the hidden potential of the 4cyl. He had the car for 2 days, so I dont think much could have been done. Crazy I know it sounds, but we also added about 450 pounds to the car as we are both approx. 220. Maybe on paper this is not possible, but has anybody experienced such from a 4cyl???Please dont flame me!!
AND no, I'm not some crazy ass 10 yr old or something(sorry for beind defensive, but I've seen some harsh remarks for some simple questions)

if youre gonna stick with a 4cyl put a custom turbo on it. otherwise youre gonna be very limited with a 4 banger

and what you refer to as "tweaking" may give the impression of extra power by just simply fixing a problem that may have been robbing full performance. you probably had some clogged or just dirty injectors and by cleaning them out made it feel a lot better