I have never dealt with domain which start with figure for example www.4xxx.com Is any problem with such domains I mean SEO and customer's memorising?
Not exactly sure from an SEO point of view, but it will be more difficult for the customer to remember unless it matches the domain well. From a resale purspective, it dramatically reduces any value of the domain completely.
However it may be good to have number 1 in the beginning if you can take advantage of this fact ... something like: 1st Online Pharmacy at 1-online-pharmacy.xyz But definitively without the number the domain is usually worth more.
domain names that starts with numbers remind me of the Yahoo directory ranking way before google born, people rush for domains with numbers trying to rank high in the yahoo listing... China has a lot of popular website using just numbers as domain name, but well that is a totally different market. Anyway, my vote for no number domain names
I don't think it should be a problem at all for SEO purpose but, it can have a big impact on memorizing part. It will be difficult to remember and you will get very few direct referrals due to that. On the other hand you can do SEO for any domain and can get ranked well in search engines.