I'm 7 w pregnant with a very small fetus barely seen on ultrasound ..hcg 4518..no pain no bleeding..should i be worried ?

Yes: Well it seems you have been waiting so you already may have your answer. 7 weeks and the HCG levels do not match. There are two scenarios possible. One is that your pregnancy is much less than 7 weeks or that your pregnancy may not be viable or normal. Either way the best approach if you are not having other problems is to continue with monitoring with ultrasound and HCG measurements.