As far as trademarks are concerned, it depends on the specific composition of ABCD. If AB is a key word and CD is a common word, it basically constitutes trademark infringement. If the combination of ABCD is significant, it should not be.
Unfair competition: See if the ABCD you use is easily confused with AB or ABEF, making people mistakenly think that it is someone else's goods or has a specific connection with others. Specifically, it should be judged according to the specific relevant words and Article 6 of the Anti-Unfair Competition Law.
Practical suggestion: if you apply for trademark registration of ABCD, it means that the competent authority does not think it is similar or confused with the trademark applied earlier, so you can use it with confidence; If it doesn't pass and the AB trademark is not cited rationally first, you should stop using ABCD, which is very risky in law.