The main difference that cannot be economically overcome is the 3.8L engine in the 4S.
The only chance for doing this with an engine swap is if you find a 4 that has every configuration and color you want, but has a blown engine that must be replaced and so its priced accordingly. Then get a 3.8L put in instead of the 3.6L that the 4 came with. The rest can be put in, or may be on the 4 already (may have PASM, replacing brakes is doable).

But if the 3.8L engine is really something you want, buy a 4S.

 

Here are a couple of tips since you are new.
The 997.1 model year is from 2005 to 2008. The 4 and 4S first model year was in 2006.
Do not buy 2005's.
Also, do not get the tiptronic. The tiptronic is programmed for fuel savings so your are likely not to be happy with it. The oil pump was not programmed for shifts at low rpm and therefore did not raise the oil pressure enough and as a result most cars with IMS problems had tiptronic. Porsche engines and oil pumps prefer to be run and higher rpms and therefore higher oil pressure.
The 997.1 model came out in 2009 and major changes were: interior upgrade, better nav, possible to get PDK, direct injection motor which added 30 hp and reduced engine parts by 40%.

The 997.2 is a much better car if you can afford it.

Repair to look out for. Run the car hot, stop it and restart immediately. If the car is slow to start, it will require a new power cable to the starter and new starter. There is a know problem with cracking and corrosion on his cable. The cable is $80 but labor is $1000.
Always have a Pre Purchase Inspection performed and a shop of your choice. You can search here for items to look for in a PPI.

 

As long as it has hips you'll be happy. After 9 months of owning my 997.1TT I still stare at them sometimes.

I had this issue on my 997.1TT. Pin through firewall was corroded and caused resistance to the point were the car wouldn't start. It did not cost $1000 however; you can get to it without upping the entire dash off (the way most approach it). It can be accessed by removing the the carpet and I think the glovebox from the passenger footwell*. I paid NZ$400 total for the repair.

*Your mechanic needs to be relatively small and nimble

 

Just some added perspective. The hips on the 4 and 4s are sweet. If they made the C2S with wide hips I'd have been all over it but they didn't. So when I got my 997 a month ago I got the C2 because it had the right color, options and price point.

I came from a 2006 Cayman S with 295 hp. So a 50hp bump to a 2011 C2 (also 5 years newer) was enough of an upgrade for me. What you want in terms of power is a little dependent on what you are used to.

Minor gripes are that my CS had red brakes and my C2 does not. Same issue your going to have from C4 to C4s. Solution is to paint them anything but red. CS had white faces gauges. CS had sports seats which I miss. C2 has memory and electronic adjust but I do miss those higher seat bolsters. I'm not sure what seats are standard between a C4 and a C4s but get seats you like. I really like the sports seats.

At some point in time you have to stop chasing power. There's always something faster and you get used to speed. That's what motorcycles are for and that why I don't chase power anymore.

As for depreciation ... I think they all take a hit. Highly optioned ones more so than others.

I think you will be unhappy if you get something slower than what you already have. Otherwise an upgrade is an upgrade but ultimately do what you 'want' if it's not a hardship otherwise you'll have that nagging feeling in the back of your head. I mean what are they 5-8k more used? Depreciate it another 5+ years and your talking losing half if that ... So 2-4k. Let's call it 3 ... so 50 bucks a month extra to keep from saying 'I shoulda ....'