You can ensure RISC OS 4 is magnified on startup by editing $.!Boot.Choices.Boot.Tasks.!Boot and adding the line WimpMode X1280 Y1024 C256 EX0 EY0 or whatever. If you need more magnification, reduce the resolution (or if only one application needs to be magnified then you may be able to adjust that). You can adjust the desktop font by opening !Boot and choosing Fonts (I use Homerton.Medium).
Alternatively, use a VNC server and magnify at the client (which has the advantage that you can share a one-input monitor without using a KVM switch).
Some people prefer to have dark backgrounds and light text. To achieve this throughout RISC OS, download my RISC OS high-contrast theme and run the HighContrast obey file. (You can also edit the source if you need to change the colours.) The NormalColours obey file can be used to temporarily switch back to normal colours, which you will sometimes need to do because some programs don't work well with HighContrast. After making each change, close and re-open any Edit windows (that way NormalColours still gives you high contrast in Edit).
If you want you can add the line Filer_Run $.high-contrast/zip (correcting the path as appropriate) into $.!Boot.Choices.Boot.Tasks.!Boot so that the options are available on startup (you could also add the line Run $.high-contrast/zip.HighContrast if you want it to be selected by default). Both of these rely on SparkFS being run first (so put the line late in the file). Some applications will display differently depending on whether they are loaded before or after HighContrast. Sometimes you will have to explicitly set the application's foreground colour to something other than black, or change the application's Choices.