If you mean converting numeric to string or vice versa, left or right padding a string, getting a substring, etc.: There are several businessfunctions available for doing this.
If you mean linking those fields in a businessview: you can't (at least not anymore). Does F0411 have an (unused) URAB column? If yes, then you could populate F0411.URAB with the numerical value of the RMK column, and then link F0411.URAB to F4211.DOCO. (I've seen one of our customers do this to link F4801.RORN to F4211.DOCO: In their old release, they were able to join RORN to DOCO in a view, but after the upgrade, the business view tool wouldn't allow joining different type columns anymore. So they use F4801.URAB now to link to F4211.DOCO (When F4801 is created, custom code populates F4801.URAB with F4211.DOCO).