In the following signs, we have 高压电 (high voltage) and 低压电 (low voltage). Chinese Converter gives the pinyin: gāo yā diàn and dī yā diàn (this has been wrong in the past). However, Dict.cn and WrittenChinese.com assign 压 two possible tones: yā (first tone) or yà (fourth tone). So I don't know which is correct.
Question: What is the correct tone for 压 in 高压电 and 低压电?