We analyse the Weak Gravity Conjecture for chiral four-dimensional F-theory compactifications with N = 1 supersymmetry. Extending our previous work on nearly tensionless heterotic strings in six dimensions, we show that under certain assumptions a tower of asymptotically massless states arises in the limit of vanishing coupling of a U (1) gauge symmetry coupled to gravity. This tower contains super-extremal states whose charge-tomass ratios are larger than those of certain extremal dilatonic Reissner-Nordström black holes, precisely as required by the Weak Gravity Conjecture. Unlike in six dimensions, the tower of super-extremal states does not always populate a charge sub-lattice.The main tool for our analysis is the elliptic genus of the emergent heterotic string in the chiral N = 1 supersymmetric effective theories. This also governs situations where the heterotic string is non-perturbative. We show how it can be computed in terms of BPS invariants on elliptic four-folds, by making use of various dualities and mirror symmetry. Compared to six dimensions, the geometry of the relevant elliptically fibered four-folds is substantially richer than that of the three-folds, and we classify the possibilities for obtaining critical, nearly tensionless heterotic strings. We find that the (quasi-)modular properties of the elliptic genus crucially depend on the choice of flux background. Our general results are illustrated in a detailed example. seung.joo.lee, wolfgang.lerche, timo.weigand @cern.ch arXiv:1901.08065v1 [hep-th] 23 Jan 2019 F Example of an emergent critical string in a Type B weak coupling limit 62 Table B.2: GLSM charges of the toric coordinates of the four-fold Y 4 that is an elliptic fibration over the base three-fold B 3 , which by itself is described in Table B.1.Here, each of the generators is described in terms of its intersection numbers with the 10 toric divisors d ρ := {ν ρ = 0}, for ρ = x 0 , · · · , s, where we keep the same ordering as in Table B.2. The (closure of the) Kähler cone K(Y 4 ) is then spanned by the divisor classes, where l (a) J b = δ a b for a, b = 1, . . . , 5. Note that three of the generators J 1 , J 2 , and J 4 of K(Y 4 ) are respectively pull-backs of the generators j 1 , j 2 , and j 3 in (B.6) of K(B 3 ), where, by slight abuse of notation, the base toric divisors and their pull-backs are denoted by the same symbols. Finally, the intersection polynomial is given as I(Y 4 ) = 10J 1 2 J 3 2 + 46J 2 2 J 3 2 + 262J 3 4 + 16J 3 2 J 4 2 + 4J 1 2 J 5 2 + 20J 2 2 J 5 2 + 134J 3 2 J 5 2 + 8J 4 2 J 5 2 + 52J 5 4 + 57J 3 3