How does one say "I'm planning to go to a meeting from two to four pm today."?

Mi planas iri al renkontiĝo de 2:00 (la dua) ĝis 4:00 (la kvara) posttagmeze hodiaŭ

Thanks! Is it common for folks to say "la dek kvara" isntead of "la dua posttagmeze"?

Certainly, you can say "la dek kvara" - it is clear, that it is pm.

Esperanto aims to be understandable everywhere, therefore the only acceptable date and time format when written with numbers only is that of the International Standardization Organisation (ISO). For instance I'm writing this at 2019-12-01 00:55, but having said that there are a lot of people, who don't understand that format. Therefore the date is usually written at least with the month spelled out la 1-a de decembro.

For time there is no such clarifying format. UEA uses the French(?) format 00h55, but not all like it. Some use the antaŭtagmeze/posttagmeze, but some find it really old-fashioned and extremely confusing when used for time just past the midnight or noon.

For written times I recommend to stick at the standard, i.e. 24-hour format with leading zeroes and colon as a separator between hours and minutes (and seconds if necessary). When speaking you can use either that ("la dek kvara" as Sergejm said) or just "la dua" (which is usually understood to mean the afternoon therefore posttagemeze sounds superfluous), if speaking informally. However if announcing a meeting, I would stick to "la dek kvara".