What size iron?

Heh... for any auto reasons.. anything below 45watt. What you need to consider is not the wattage of it but how hot the actuall tip temp is. Will it get hot enough to melt solder that melts at 480degreeze? Hell just about any iron will..... problem comes in when you get an iron without a controll knob, or on/off switch... theres no way of cooling it down... or preventing it from getting too hot.

So yeah.. if that made any sense.. If i was you, i would buy a portable butane model for about the same price...

Price range is about 20 bucks or less, for more I could get all the bells and whistles but I'll really only use it a few times. The ultimate on/off switch is the plug, they all have one unless i drop for a cordless.

  Is it possible to solder 4ga?

I prefer butane irons as well.  If you were to want to solder 4 gauge using an electric iron for $20 or less check out ebay:

http:///.com/dll?ViewItem&item=3655154737&category=4770

To directly answer your question, Yes it will solder 4gauge wire. Remember, Heat, time, mass is the equasion. Does the soldering iron get hot enough to melt the solder? Yes it does, now the only thing directly affected by this is "Time". With a cheapie crap iron, it will take a considerable ammount of time to heat a 4gauge joint up enough that it will melt solder, but with a top of the line snapon 125watt soldering gun, you will heat the joint up fast, but also run the risk of burning alot of sh*t and most importantly, burning the crap out of your solder.

Best bet is to go to sears and buy the "weller partasol" portable soldering iron. Head to your local cigar/cigarette shop and buy 2 large cans of butane for 2$ and that will last you a LONG ass time. I know with my T110K Matco, 2 cans lasted me about 2 weeks of 2-3 installs per day and the iron was running constantly each day. In other words... it uses very little butane.

Please reffer to my "How to solder, beginners guide" in the general section at the top for the basics. I honestly feel that once a person learns how to solder correctly, nothing else will ever be good enough for them(ie~Butt connectors, ttaps, twist/tape). Then you can invest into a few different models (AC/Butane/Battery/Iron/Gun) and never ever ever be in need of a connector assortment again ~ Maybe a spade connector, but you will solder that on anyways.

I read the how to solder post (great!) and it got me thinking about it, so I looked around ebay (my favorite place, almost my whole system is off ebay) and saw irons up to 200 watts and down to 30 or so. I am considering the big iron w/ a temp controller...little over budget. I am also eyeballing the butane irons, but I figure a cheap one will be worse than a corded iron of similar price.

Actully... much to my suprise.. ok maybe not... most of the models you can buy from high end companies such as Snapon/Mac/Matco are all pretty much made by the same company(s).. knowing what i know now.. i coulda saved big time on the models i own, but again.. i woulda had to pay em off in full 1st.

For instance, Weller makes the twin of Matco's PPSK and T110K with a few differences, such as the name on the handle and the case it comes in. Whal makes the twin brother for the Snap-On cordless, battery operated model and sells alot cheaper as well. Identical except the name on the tool and the perks like paying 20$ a week on it.

So in other words, if you decide to go with a cheaper model, it doesent mean your going with something of less quality, your just not paying for the name on the handle...

Just like Craftsman VS Snap-On hand tools. Same manufacturer, different name on the handle, different price.. and i must admit... its nice taking a shattered socket into sears and getting a new one.. even tho you beat the sh*t out of it with a ballpeen hammer trying to get something out of it... snappy would consider that excessive abuse and try to sell you the 70$ replacment.. but thats a different post all together