I'm not sure what you consider good. New Jersey is one of the most expensive states to live in the United States.
rent.com had this to say - "with an overall cost of living 32% above the national average, and housing a whopping 73% above the average."
$4500/month after taxes and other things is probably $3000/month. Your rent is probably going to be at $1000/month or more. That leaves $2000/month for everything else - food, utilities, insurance, etc. In my opinion a software developer especially one with 4+ years of experience should not have to struggle to make ends meet. But if people keep taking jobs far below market value - the companies will keep getting richer and the people poorer.
According to CNN/Money the median house price in Newark is $369,500. In Bergen-Passaic it is $423,500. You would need 74,000 in Newark and 84000 in Bergen-Passaic to have 20% down on a house. If you managed to save 1K a month it would take 7 years to buy a house - by which time the houses would be more and it would take more like 10 years.
I made this much when I lived in Indiana and it was a fair wage for the location. I moved to Florida and needed to make alot more. I saw numbers that suggest New Jersey is 30% more expensive to live then were I'm at in Florida.
I know nothing about the UK but I have a feeling the London numbers aren't going to be much better.
London is one of the most expensive places to live on earth right now. �30,000 a year is very little for London. You will probably have to share a flat at that salary.
Is it doable? Probably. You will probably net �1800 a month. Monthly rent will be a minimum of maybe �500 a month including council tax, transport another �180 or so, food maybe �500, utilities �100. That is if you eat at home. London has the most expensive restaurants and pubs on earth so don't expect to eat out on that too much.
Surely that figure for London must be after tax? If not, it's quite low for someone with your experience. I've only 18 months experience and I'm on more than that. You can get a rough idea of current salaries based on your skill set from www.itjobswatch.co.uk.
London is a fantastic city though, and I'd whole heartedly recommend it.