Somebody wants me to create a web page with a DOC Type of HTML 4.01

I've only ever coded using "xhtml transitional" coding tags. Xhtml is of course a stricter standard. Tags need to be lower case, opening tags and closing tags matched etc.

In xhtml non-balanced tags like break and image tags need the trailing backslash ie. <br /> ,<img ..../>
But in HTML these are not necessary (unacceptable??).

What i was wondering is if the xhtml coding will fail or cause validation problems in a HTML 4.01 document.

Yes, but its best to "code" in XHTML 1.0 and prepare for HTML 5.
http://whatwg.org - HTML 5 Working Group

HTML5 is AT LEAST 10 years away from being a almost usable standard, and if it progresses at the same rate as CSS2 has done with browser compliance, probably nearer 20 years.

In xhtml non-balanced tags like break and image tags need the trailing backslash ie. <br /> ,<img ..../>
But in HTML these are not necessary (unacceptable??).

So, use HTML 4.01 STRICT. Validation is not the end-all, it is a TOOL. It's not going to make any real difference if the validator complains about XHTML's closing tags, as long as your code is correct in the first place.