\[ \boxed { \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{4}- \left ( y \left ( x \right ) -a \right ) ^{3} \left ( y \left ( x \right ) -b \right ) ^{2}=0} \]
Mathematica: cpu = 0.701589 (sec), leaf count = 383 \[ \left \{\left \{y(x)\to \text {InverseFunction}\left [-\frac {\sqrt [4]{a-\text {$\#$1}} \sqrt {\frac {\text {$\#$1}-b}{a-b}} B_{\frac {a-\text {$\#$1}}{a-b}}\left (\frac {1}{4},\frac {1}{2}\right )}{\sqrt {b-\text {$\#$1}} \sqrt [4]{\frac {a-\text {$\#$1}}{a-b}}}\& \right ]\left [c_1-\sqrt [4]{-1} x\right ]\right \},\left \{y(x)\to \text {InverseFunction}\left [-\frac {\sqrt [4]{a-\text {$\#$1}} \sqrt {\frac {\text {$\#$1}-b}{a-b}} B_{\frac {a-\text {$\#$1}}{a-b}}\left (\frac {1}{4},\frac {1}{2}\right )}{\sqrt {b-\text {$\#$1}} \sqrt [4]{\frac {a-\text {$\#$1}}{a-b}}}\& \right ]\left [c_1+\sqrt [4]{-1} x\right ]\right \},\left \{y(x)\to \text {InverseFunction}\left [-\frac {\sqrt [4]{a-\text {$\#$1}} \sqrt {\frac {\text {$\#$1}-b}{a-b}} B_{\frac {a-\text {$\#$1}}{a-b}}\left (\frac {1}{4},\frac {1}{2}\right )}{\sqrt {b-\text {$\#$1}} \sqrt [4]{\frac {a-\text {$\#$1}}{a-b}}}\& \right ]\left [c_1-(-1)^{3/4} x\right ]\right \},\left \{y(x)\to \text {InverseFunction}\left [-\frac {\sqrt [4]{a-\text {$\#$1}} \sqrt {\frac {\text {$\#$1}-b}{a-b}} B_{\frac {a-\text {$\#$1}}{a-b}}\left (\frac {1}{4},\frac {1}{2}\right )}{\sqrt {b-\text {$\#$1}} \sqrt [4]{\frac {a-\text {$\#$1}}{a-b}}}\& \right ]\left [c_1+(-1)^{3/4} x\right ]\right \}\right \} \]
Maple: cpu = 0.140 (sec), leaf count = 141 \[ \left \{ x-\int ^{y \left ( x \right ) }\!{\frac {1}{\sqrt [4]{ \left ( { \it \_a}-a \right ) ^{3} \left ( -b+{\it \_a} \right ) ^{2}}}}{d{\it \_a} }-{\it \_C1}=0,x-\int ^{y \left ( x \right ) }\!{-i{\frac {1}{\sqrt [4]{ \left ( {\it \_a}-a \right ) ^{3} \left ( -b+{\it \_a} \right ) ^{2}}}}}{ d{\it \_a}}-{\it \_C1}=0,x-\int ^{y \left ( x \right ) }\!{i{\frac {1}{ \sqrt [4]{ \left ( {\it \_a}-a \right ) ^{3} \left ( -b+{\it \_a} \right ) ^{2}}}}}{d{\it \_a}}-{\it \_C1}=0,x-\int ^{y \left ( x \right ) }\!-{\frac {1}{\sqrt [4]{ \left ( {\it \_a}-a \right ) ^{3} \left ( -b+{\it \_a} \right ) ^{2}}}}{d{\it \_a}}-{\it \_C1}=0,y \left ( x \right ) =a,y \left ( x \right ) =b \right \} \]