I think rūpa is a term that alludes to way more than just the 4 elements…
Rūpa,(nt.) [cp.Vedic rūpa,connected etymologically with varpa (Grassmann).-- The Nom.pl.is rūpā & rūpāni] form,figure,appearance,principle of form,etc.-- A.
Definitions.(…) “form,” and as better (philosophical) terms “matter,” “material quality” are recommended). (…) form, either contrasted with what is unseen,or taken for both seen and unseen, representations, statue, beautiful form,beauty, (…) drawing,or arts & craft,(…) form,like,kind,of a certain condition or appearance.
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In all these representations of rūpa we find that an element of moral psychology overshadows the purely philosophical & speculative aspect.
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2.(metaphysically) as the representative of sensory or material existence:(a) universally as forming the corporeal stratum in the world of appearance or form (rūpa- bhava) as compared with the incorporeal (arūpa-bhava),being itself above,and yet including the kāma-bhava.(The kāmabhava is a subdivision of rūpabhava,which has got raised into a third main division.) This triad is also found in combns with loka or dhātu (see dhātu 2 a & d),or avacara.
(…) in the making up of the individuality as such (nāma-rūpa),where in contrast with nāma (as abstract,logical,invisible or mind-factor) rūpa represents the visible (material) factor,resembling kāya (cp.phrase nāma-kāya in same sense).
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The following are current defns of nāma-rūpa:
nāma-(kāya)=vedanā,saññā,cetanā,phassa,manasikāra
rūpa(–kāya)=cattāro mahā-bhūtā catunnaṁ m-bhūtānaṁ upādāya rūpaṁ (otherwise kāya-saṅkhārā)