According to the current provailing grammars, as well as 现代汉语词典第五版（ The comtemporary Chinese Dictionary, 5th edition), 成 here is still a verb. Actually in modern Chinese, many verbs like 成 can be used following another verb. For 成, you'll see 切成，打成，变成，编成，看成，说成... ---- lots of phrases in this pattern. The meaning for the construction 'verb sth/sb 成' is ' verb sth/sb and make it become...' This pattern is named as 连动式 （serial verb construction) in Chinese. Owing to lack of morphology, Chinese sentence structure is distinct from your language, so in a sentence there can exist two or more verbs which are all 'naked'. Another common construction containing two parallel verbs is ' pivotal construction 兼语式', for example, 我老婆叫我去洗澡：here in this sentence, '叫'我'去洗澡' as a whole is pivotal construction, and ‘去' '洗澡' makes a serial verb construction.

you'd better consider it a fixed collocation...... in my opinion, and according to my casio, 成 is never a preposition, and in this situation(变成、切成、涂成、写成...) it should be a verb which means become...and the word you consider a verb(变、切、涂、写....) is just used to demonstrating how you make that thing become... for example...把蛋糕切成四块=turn the cake into 4 pieces by cutting it.

现代汉语用词很不准确，像你说的两个句子： 把 蛋糕 切 成 四块 (to cut cake INTO 4 pieces) 把 英文 翻译 成 汉语 (translate english INTO chinese) 其实，这里的“成”更为准确的替换词应该是“为”。这样“为”(wei)才是preposition。