Combining numbers with letters isn't looked upon favorably. For example fly2orlando would have a lesser value than flytoorlando. That being said, 4rxz isn't as close to forex as maybe 4x. Now 4x would have been a better buy as it is still easier to remember and type in. Besides, it is very close in the pronunciation aspect and comes in the 2 letter category.
The other point would be the letters 'x' and 'z'. These letters devalue the worth of a domain as there are very few word combinations possible if we were to use acronyms.
All in all, I would estimate the worth of this domain to be around low XX . I may be wrong here, so more expert valuation should be taken into consideration.