I have a 2003 Nissan Altima 3.5 V6 and today my radiator was leaking from the bottom. I drove it to the mechanic and at about 4.5 miles from the mechanic the car temperature gauge was all the way to the top at H. Is it possible that I could have damaged the head gasket or warped something for driving it like that for 4.5 miles ??
If the gauge went all the way to the "H" you have no idea how hot the motor actually got because the gauge was at it maximum. If you had a leak the cooling system was low on coolant. the more coolant lost the hotter the engine ran. There is a good chance some damage was done from the motor overheating. There are tests your mechanic can do to confirm a bad head gasket. The only way to confirm if the heads are warped is to remove the heads and measure them. There will be some labor charges just to determine the extent of the damage.
Yes, overheating the engine can damage head gaskets and warp or crack the head(s) (4 cylinders have one head). On these types of engines, the heads mounts to the top of the engine and are (typically) made of aluminum. The engine, on the other hand, is made of cast iron and is very thick and heavy. So, when the engine gets too hot it can withstand the overheating and resists warping and cracking, while the head tends to warp/crack under too much heat. Fixing this can be a $1,500+ job at a shop and you're probably looking at $600-$700 job on your own after you pay to have a shop plane the head (and check for cracks) and buy the gaskets and head bolts. But it's not a terrible job and can be done on weekend if you're inclined to do so. If the head isn't warped or cracked but maybe you just blew a head gasket, then you can probably get away with fixing it for a couple hundred at a shop.
The shop can sometimes test if the heads/gaskets are warped or cracked by putting pressure on the coolant system without the engine running. Other shops may want to charge you $1,000 just to tear into it to see if it's the head/gasket.
The risk you run by just adding coolant and watching for leaks are the obvious a) you risk that there is a head and or gasket leak and you may be left stranded, b) if there is a leak and water gets in the combustion chamber then you risk (what's called) throwing a rod and then you'll be looking for a used engine and at least $3,000 - $4,000, or c) if some coolant gets into the oil it can rust bearings and shorten the life of your car. I would guess that there's probably 50 more scenarios that could happen, but these ones would be bad and are not atypical.
If you're feeling lucky (or poor?) and if it were me I would probably just have the shop fix the radiator and keep an eye on the temp and coolant level. They're not going to give you the car back if it's leaking coolant all over the place due to a cracked gasket or head anyway ..so they'll have a decent idea if it is once they fill it up with coolant and run the engine. Also, keep your nose out for the smell of coolant which smells almost exactly like maple syrup. If you see yellow under your car or smell maple syrup, then get it to someone you trust.
Hope this helps.