Check out the Sticky'd threads. They will have a lot of useful information and pointers. Also search is your best friend. A lot of people will give you hard time for not searching things first.
I would say for exterior (in no order)...
1. Tint
-25% rear 35% front
- or whatever you prefer
2. Lower on some coils
-Megan EZ streets run for around $700, last I checked
-they have a really nice drop to get rid of the fist gap we have between our tire and fender
- I think springs are a waste of time because your struts will eventually blow and you can either get new one from honda which will cost 1k and you cant adjust the height or damper (softness or stiffness) of your ride.
3. Wheels
- I think some great wheels are
- the HFP 19s around $1000 + tires
- Varrstoen (Volk TE37 Reps) 19s for around $1000 + tires
- I would say get 19 or bigger, the 18's dont seem right in the giant wheel well
4. trunk lip spoiler
-many of the forum sponsors could get it for you for around $100
5. HID headlights
- Retros are great around $120 with shipping
- DDM were cheap but are going out of business last I heard they were around $35 bucks
... I think that would be sufficient for the exterior
As far as performance....
- I really don't think performance modifications on an i4 are worth it because the gains you will see will be very minimal, and you should have gone for a v6 if you wanted some more horsepower
I dont know much about the perfromance mods for a i4 but I guess you could get a Cold Air Intake(CAI) or Short Ram Intake (SRI), a few companies are...
- Takeda
- Fujita
- AEM
- idk prices
good luck. welcome. have fun.