To clear the air, before I advise you on whether or not to consider dropping thinking skills, try researching more into university requirements for A Levels and to be specific- degree entry standards. Although you're contemplating on dropping Thinking Skills, you should know that 'Thinking Skills' at A Level stage is usually denied by most mainstream universities as a substitute qualification in place of a possible THIRD A Level. This could lead to a detrimental choice of subjects and possibly a weak form of knowledge into the degree you could be applying for. Thus, from a general point of view had you cancelled out for e.g. Geography, the chances of you getting accepted would have been slim but knowing you have compromised it as a fourth A Level option, from what I know you can be accepted quite easily. Also, the fact that you already have three major subject choices which are classified as 'strong' A Levels just puts you at more of an advantage into getting enrolled.
There are many students who only have x3 A Levels maximum. With this being fact, as much as you'd like to think of this being less workload, this is a mere understatement as scads of people have coped well with 4 A Level options in their first and second year at sixth-form or college. Ultimately, it just depends on who you are when it comes down to your education as some people may see 4 subjects as a balance to their academia, whereas others may validate this many subjects as a cause for stress and might opt for only 3 subjects.
The reason as to why Thinking Skills AS/A2 isn't accepted by most universities is solely due to the factor of it not being able to suffice enough information in association to any degree program and is usually seen as a means of surviving the UCAS evaluation stage of getting into university.
I'd say that Thinking Skills is just an advanced form of data handling and problem solving which you are expected to have already mastered up to this stage of your academic life. If this is not the case, your common sense and experience at GCSE should reflect upon this judgement.)