I have to present my stimuli for a fixed period of time (4 seconds),
I tried by adding /ontrialbegin= [trial.mytrial.stimulustime(4000)] inside my trial, but every time I try to run it, it gives an error message. 
What should I do?

If your trial doesn't require a response, simply set its /trialduration to 4000.

<trial example>
/ stimulustimes = [0=yourstimulus]
/ validresponse = (0)
/ trialduration = 4000
</trial>

If your trial requires some kind of response after the stimuli have been removed, do

<trial example>
/ stimulustimes = [0=yourstimulus; 4000=erasescreen]
/ validresponse = (57)
</trial>

with

<shape erasescreen>
/ shape = rectangle
/ size = (100%, 100%)
/ erase = false
/ color = white
</shape>

See https://www.millisecond.com/support/docs/v4/html/howto/howtoerase.htm

Thank you so much! 

I need a response so I tried with the second one but it gives error messages saying "Stimulus sequence already defined". 

Then you're doing something wrong. What that is, I cannot tell you without you providing your actual code. Note: Your <trial> elements may only have either a single /stimulustimes attribute or a single /stimulusframes attribute. You may not have both in a given <trial> element, and you may not have multiple /stimulustimes or /stimulusframes attributes in a given <trial> element either.

Yeah, I noticed I already had a /stimulusframes attribute so I substitute it with /stimulustimes.

Thank you so much

The problem now is that to continue with the presentation of my stimuli I have to press the response key, while I want the stimuli to be presented one after the other and to press the key only to give the response.