Re: Rising HCG levels but nothing on u/s...
4 weeks is definitely too early to be seeing anything (unless your dates are off). At 5-6 weeks you may see the sac and possibly fetal pole, but you may also see nothing still.
By 6.5-7 weeks, you would see the wee bean in there and a heartbeat as well.
If you can put off the appt, I would wait another week, until closer to 7 weeks. That one would "tell" you much more than early ones. Although, if there's something else they're watching for, with the early ones, than I'd do those as well, just not expect to see anything until later on.