Catalysis of the reaction: gellan = n beta-D-4-deoxy-delta4,5-GlcAp-(1->4)-beta-D-Glcp-(1->4)-alpha-L-Rhap-(1->3)-beta-D-Glcp. This reaction is the eliminative cleavage of beta-D-glucopyranosyl-(1->4)-beta-D-glucopyranosyluronate bonds of gellan backbone, releasing tetrasaccharides containing a 4-deoxy-4,5-unsaturated D-glucopyranosyluronic acid at the non-reducing end; in the product, the abbreviations are D-glucose (Glc), D-glucuronic acid (GlcA), and L-rhamnose (Rha).