Do there exist functions $F,G$ on $[0,1]$ with $0\le F,G< 1$, such that for all $x, y\in [0,1]$ with $x+y\le 1$, the following hold?


1) $G(x)\le x$,


2) $G(1)<1$,


3) $F(x)>0$ if $x>0$,


4) $\min(y,F(x)) \le G(x+y)-G(x)$.



Simple but entangled inequalities
$\newcommand{\de}{\delta}
\newcommand{\vp}{\varepsilon}$


No such functions $F,G$ exist.


Indeed, let $\vp\_x:=F(x)$, so that, by property 3), $\vp\_x>0$ for all $x\in(0,1]$. Take any $\de\in(0,1)$ and let 
\begin{equation}
 E:=E\_\de:=\{x\in[\de,1]\colon\forall y\in[\de,x]\ \, G(y)\ge G(\de)+y-\de\}. 
\end{equation}
Note that $\de\in E$. So, $E$ is a nonempty interval of the form $[\de,s]$ or $[\de,s)$ for some $s\in[\de,1]$. 


If $E=[\de,s)$, then $s>\de$, because $E$ is nonempty. 
Also, by property 4), $G$ is nondecreasing. So, if $E=[\de,s)$, then 
\begin{equation}
G(s)\ge G(s-)\ge\lim\_{y\uparrow s}[G(\de)+y-\de]=G(\de)+s-\de, 
\end{equation}
so that $s\in E$ and $E=[\de,s]$. Thus, in all cases, $E=[\de,s]$. 


If $s\ne1$, then $s\in[\de,1)\subset(0,1)$. So, $\eta\_s:=\vp\_s\wedge(1-s)>0$ and, by property 4), for all $h\in(0,\eta\_s]$ we have $G(s+h)\ge G(s)+h\ge G(\de)+s-\de+h$, so that $s+h\in E$ for all $h\in(0,\eta\_s]$, which contradicts the fact that $E=[\de,s]$. 


So, $s=1$, $E=[\de,1]$, and hence $G(1)\ge G(\de)+1-\de\ge1-\de$. Since $\de$ was an arbitrary number in $(0,1)$, we have $G(1)\ge1$, which contradicts property 2). 


Thus, no such functions $F,G$ exist. One may also note that property 1) was not needed or used here. 


