ADHD = Attention Deficit Hyperactivity Disorder
ADD = Attention Deficit Disorder (ie, without the hyperactivity)
ADDD - no idea, not heard of it
ASD = Autistic Spectrum Disorder(s).
Autism is now the one diagnosis given to cover any person who is on the Autistic Spectrum.
Autism is completely different from ADHD / ADD (though people can have both conditions).
ADHD is rarely diagnosed before about 6 or 7, as so many of the behaviours are fairly typical in toddlers / little ones anyway. It needs to be separated out from just immaturity.
Not sure it is a good idea to start Googling.
The assessment process is different in different parts of the country, but what they all have in common is a long wait!
What is useful, if for you to start thinking about how you perceive your ds to be 'different', or, even in what way "people" tell you he is different. The paediatrician will have a limited timescale to get to know as much as possible about him - the more you have thought about in advance, the clearer the picture will be.
Include things like "At Nursery, they say he ......" even if you don't see it at home. As much as possible, try to 'quantify' what he can do / does / doesn't do - how long does he do X for / how often does Y happen / what was happening before Z happened / etc. Rather than 'doesn't sleep well', write down the pattern' for a typical night. Rather than 'is fussy with food' write doesn't what he does eat over a 3 day period - or week if you have time.