Be careful about passing it off as summer work. The new employer could look for a reference from that company. If they rang up your manager in the 4 month job, they could explain the real reason why you left.
Personally I think it is safer to leave the work off the CV. It is a pity, because the experience you gained may help you to get other jobs.
Another alternative is to ask the old employer not to state the real reason why you left, when giving a telephone reference to a new employer. Some employers might do you a favour, and not mention that you didn't work out. Other employers may be very honest and may want to state in a reference that your performance was not up to their standard.
I left a job recently where I had to work long hours and I was being bullied. I was there 6 months. It was a small company and even though I worked hard, it was never enough. I gave 1 weeks notice as I was paid weekly. The company expected a months notice even though they wanted me immediately when I got the job. They would barely let me give 2 wks notice to the previous employer even though I was there 2 yrs.
The agencies and new employers wanted a reference from that company. I had to make up some story about going off travelling for a while, and the company not giving me the time off. Then I had to stay unemployed while I was 'supposed' to be off travelling. Now that I am in a new company (contract work), I am considering leaving the 6month bad experience off my CV altogether, and just saying that I went travelling.
It isn't very easy to get out of a nightmare job, as the new employers and agencies query why you left every job.
For future reference, if a job isn't working out it is best to leave after 2 or 3 months, or even less time. That way the gap is easier to cover.