pretty good, loud and clear without much distortion at high volumes. lacks in bass but it's expected (it's a phone speaker, doesn't have a subwoofer) but not tinny at all
i'd say the speaker is comparable to flagship phones, without much significant difference
as for 4g, it's slower than the current 3g model
the current 3g model scores 28-29k on antutu, 4g model (estimated) would score 21k
for comparison, galaxy s3 scores around 16k, s4 around 26-27k
it depends on your needs. i don't engage in high speed downloads, and i have only 3GB anyway, 3g is enough for me. it's just that sg 3g coverage is horrible sometimes, so you have to get used to that (e.g. for my m1, there's always a blind spot between jurong mrt and clementi mrt)